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Table of Contents

Preface Acknowledgements Introduction


1.1 Rubber—Natural and Synthetic 1.2 Polyvinyl Chloride 1.3 Polyurethanes 1.4 Acrylic Polymers 1.5 Adhesive Treatment 1.6 Radiation-Cured Coatings 1.7 References


2.1 Materials and Trends 2.2 Textile Fibers 2.3 Spinning 2.4 Woven Fabrics2.5 Knitted Fabrics 2.6 Nonwoven Fabrics 2.7 Reference 2.8 Bibliography


3.1 General Features 3.2 Knife Coating 3.3 Roll Coating

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3.4 Dip Coating 3.5 Transfer Coating 3.6 Rotary Screen Printing 3.7 Calendering 3.8 Hot-melt Coating 3.9 References


4.1 General Characteristics 4.2 Tensile Strength 4.3 Elongation 4.4 Adhesion 4.5 Tear Resistance 4.6 Weathering Behavior 4.7 Microbiological Degradation 4.8 Yellowing 4.9 References


5.1 Rheological Behavior of Fluids 5.2 Rheology of Plastisols 5.3 Hydrodynamic Analysis of Coating 5.4 References


6.1 Clothing Comfort 6.2 Impermeable Coating 6.3 Breathable Fabrics 6.4 References


7.1 Synthetic Leather 7.2 Architectural Textiles 7.3 Fluid Containers 7.4 Tarpaulins 7.5 Automotive Air Bag Fabrics 7.6 Carpet Backing 7.7 Textile Foam Laminates for Automotive Interiors 7.8 Flocking 7.9 References

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8.1 Fabrics for Chemical Protection 8.2 Thermochromic Fabrics 8.3 Temperature Adaptable Fabrics 8.4 Camouflage Nets 8.5 Metal and Conducting Polymer-Coated Fabrics 8.6 References


9.1 Coating Mass Per Unit Area 9.2 Degree of Fusion/Curing of Coating 9.3 Blocking 9.4 Coating Adhesion 9.5 Accelerated Aging 9.6 Flexibility—Flat Loop Method 9.7 Damage Due to Flexing 9.8 Abrasion Resistance 9.9 Test For Colorfastness to Dry and Wet Rubbing 9.10 Low Temperature Bend Test 9.11 Low Temperature Impact Test 9.12 Cone Test 9.13 Resistance to Water Penetration 9.14 Air Permeability 9.15 Water Vapor Permeability 9.16 Resistance to Permeation by Hazardous Liquid Chemicals 9.17 Resistance to Penetration/Permeation of Chemical Warfare

Agents 9.18 Resistance to Penetration by Blood-borne Pathogens 9.19 Electrical Resistivity of Fabrics 9.20 Reference

Appendix 1 Appendix 2

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COATED textiles applications are found in defense, transportation, healthcare,architecture, space, sports, environmental pollution control, and many other

diverse end-product uses. I developed an insight into the breadth of the subjectduring my long association with the Defense Materials and Stores Researchand Development Establishment (DMSRDE, Kanpur, India) while working onthe development of protective clothing and related equipment. The opportunityto visit and work at several coating facilities has given me a feel for the com-plexity of the coated textile industry. The world production of coated fabricsused for defense alone every year is on the order of several billion dollars. Ex-tensive research is being done on a global basis, and many new products, suchas breathable fabrics, thermochromic fabrics, and charcoal fabrics, are enteringthe market. The subject is spread over a wide range of literature in polymerscience and textile technology, with no single comprehensive book available.The motivation to write this book was to fill this void and to create a generalawareness of the subject. This book is meant for scientists and technologists inacademic institutions as well as in the coating and textile industry. The purposeof this book would be served if it could create additional interest in the coatedtextile industry and stimulate R&D activity to develop newer and better coatedtextile products.

A.K. SenKanpur2001

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I had a long tenure with Defense Materials and Stores Research and Devel-opment Establishment (DMSRDE), Kanpur, India, as a research scientist,

which has recently ended. DMSRDE, Kanpur, is a premier research establish-ment under the Defense Research and Development Organization (DRDO),Government of India, and is primarily responsible for research and develop-ment in nonmetallic materials required for the defense forces. The scope ofR&D activity of the establishment encompasses a wide range of scientific dis-ciplines including polymers, composites, organometallics, lubricants, anticor-rosion processes, biodegradation studies, textiles and clothing, tentage, andlight engineering equipment. It is one of the best equipped laboratories withvarious sophisticated analytical instruments.

The Department of Science and Technology (DST), Government of India,granted me a project to write a book on “Coated Textiles and their Applications.”I am grateful to DST for this opportunity. I am also grateful to Professor G. N.Mathur, Director, DMSRDE, for permitting me to continue as Emeritus Scientistand for providing me access to a library and other facilities. His encouragementand help have been a source of inspiration. I am particularly thankful to mycolleague Mr. N. Kasturia who first suggested that I write a book on the subjectand for the help rendered at different stages in writing the book. I am indebted toDrs. V. S. Tripathi, L. D. Kandpal, Messrs. Anil Agrawal, T. D. Verma, DhannuLal, Darshan Lal, R. Indushekhar, G. L. Kureel, Miss Subhalakshmi, and sev-eral other colleagues at DMSRDE for their spontaneous help, suggestions, andinput on specialized subjects. I also wish to acknowledge useful discussionsand literature provided by Mr. A. K. Mody, Entremonde Polycoaters, Mumbai;Mr. M. L. Bahrani, Southern Group of Industries, Chennai; Mr. A. Narain,Swastik Rubbers, Pune; Mr. M. K. Bardhan, Director, SASMIRA, Mumbai;Professor P. Bajaj, I.I.T., Delhi; and Professor A. Nishkam, Principal, GCTI,Kanpur. Above all, I am thankful for the encouragement, inspiration, and valu-able suggestions given by my wife, Sutapa.

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THE use of coated textiles for protective clothing, shelters, covers, liquid con-tainers, etc., dates back to antiquity. Historically, the earliest recorded use

of a coated textile was by the natives of Central and South America, who ap-plied latex to a fabric to render it waterproof. Other materials such as tar, rosin,and wax emulsions have been used over the years to prepare water-resistantfabrics. Due to their vastly superior properties, rubber and other polymeric ma-terials have become the preferred coatings. Today, coated fabrics are essentiallypolymer-coated textiles. Advances in polymer and textile technologies have ledto phenomenal growth in the application of coated fabrics for many diverse enduses. Coated fabrics find an important place among technical textiles and areone of the most important technological processes in modern industry.

Textiles are made impermeable to fluids by two processes, coating and lami-nating. Coating is the process of applying a viscous liquid (fluid) or formulatedcompound on a textile substrate. Lamination consists of bonding a prepreparedpolymer film or membrane with one or more textile substrates using adhesives,heat, or pressure. Fibrous materials are also used for reinforcing polymeric ma-terials to form composites for use in tires, conveyor belts, hoses, etc. The scopeof this book has been restricted to coated and laminated textiles and does notaddress polymer fiber composites.

Several methods of production are used to manufacture a wide range ofcoated or laminated fabrics. Broadly, they are spread coating, dip coating, meltcoating, and lamination. They not only differ in the processing equipment used,but also in the form of polymeric materials used. Thus, paste or solutions arerequired for spread coating; solutions are required for dip coating; and solidpolymers such as powders, granules, and films are required for melt coatingand lamination. The basic stages involved in these processes include feed-ing the textile material from rolls under tension to a coating or laminatingzone, passing the coated fabric through an oven to volatilize the solvents andcure/gel the coating, cooling the fabric, and subsequently winding it up intorolls.

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The properties of a coated fabric depend on the type of polymer used andits formulation, the nature of the textile substrate, and the coating method em-ployed. The subject of coated textiles is thus interdisciplinary, requiring knowl-edge of polymer science, textile technology, and chemical engineering. Theorganization of this book is based on these considerations.

Among the various polymers used for coating and laminating, three classesare mainly used for coating: rubber, polyvinyl chloride, and polyurethane. Thesepolymeric materials are specifically formulated with additives and compoundedinto a paste suitable for coating. The production of a polymeric coating fluidis one of the most important functions of the coating industry. The chem-istry of these polymers, the additives used, and their processing for coatingcompounds and fluids have been described in Chapter 1. Conventional sol-vent coatings are losing favor, as they lead to environmental pollution. Sev-eral alternative processes, such as the ecofriendly aqueous polyurethane andradiation-cured coatings, are included in this chapter. The various adhesivetreatments for improved elastomer-textile bonding have also been discussed inChapter 1.

For many years, cotton was the primary fabric used for coating; however, to-day’s coating industry uses diverse substrates made of rayon, nylon, polyester,polyester-cotton blends, and glass fibers that may be produced in woven, knit-ted, or nonwoven constructions. The physical properties of a coated fabric areaffected by the nature of the fiber and the construction of the textile substrate.The choice of the substrate depends on the application of the material. Chapter 2discusses the different fibers and their conversion into textile materials of vari-ous constructions used in the coating industry. The coating methods employedby the industry are discussed in Chapter 3. Emphasis is placed primarily on theprinciples, rather than on the engineering aspects, of the machinery. Chapter 4describes the changes that occur in the physical properties of a fabric when itis coated. A brief account of rheological factors affecting the coating has beenpresented in Chapter 5. Thus, the raw materials, the coating methods, and theproperties of the end product are presented in chronological sequence.

The large, ever-increasing variety of applications of coated fabrics is cov-ered in the three subsequent chapters. Protective clothing for foul weather isone of the major applications of coated fabrics. In the last two decades, par-ticularly after the development of GORE-TEX® laminates, there has been anexplosion of development in breathable fabrics. Chapter 6 discusses all typesof coated fabrics for foul weather protection with special emphasis on the de-velopments in the field of breathable fabrics. Coated textiles used in syntheticleather, upholstery fabrics, fabrics for fluid containers, backcoating of carpets,and architectural textiles are discussed in Chapter 7.

In various applications of coated fabrics, a functional material such as dye,pigment, or carbon is applied on the textile materials with polymeric binders.These fabrics are being used as camouflage nets, thermochromic fabrics,

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protective clothing for toxic chemicals, etc. This specialized category of coatedfabrics is included in Chapter 8. Metal coatings are finding newer uses in EMI-RFI shielding and radar responsive fabrics. These fabrics are also discussed inthis chapter.

The test methods pertaining to coated fabrics have been discussed inChapter 9. The references are given at the end of each chapter. Properties ofcommon polymers used for coating are separately provided in Appendix 1, anda few typical formulations are given in Appendix 2.

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Polymeric Materials for Coating



IN ancient times, Mayan Indians waterproofed articles of clothing and foot-wear by applying (coating) gum from a tree (rubber tree) and drying it over

smoke fires. Modern day history of coating rubber on fabrics dates to 1823,when the Scotsman Macintosh patented the first raincoat by sandwiching alayer of rubber between two layers of cloth [3].

Since then, there have been great advances in rubber-coated fabric technol-ogy. Coated fabrics are now used for diverse applications. Almost all types ofrubbers are used for coating, but the discussion here will be restricted to themore popular kinds.

Rubber is a macromolecular material that is amorphous at room temperatureand has a glass transition temperature, Tg , considerably below ambient. Rawrubber deforms in a plastic-like manner, because it does not have a rigid networkstructure. It can be cross-linked by vulcanization to form an elastomer with theunique ability to undergo large elastic deformations, that is, to stretch and returnto its original shape. For natural and most synthetic rubbers, vulcanization isaccomplished with sulfur.

Elastomers that have stereoregular configuration and do not have bulky sidegroups or branching undergo crystallization. Crystallization cannot occur abovemelt transition, Tm . The rate of crystallization is greatest at about halfwaybetween Tg and Tm . For natural rubber, for example, this is about −25◦C.The crystallites embedded in the elastomeric matrix act as physical cross-links,like reinforcing fillers. Most importantly, crystallinity can be induced by stress.Formation of crystallites enhances the strength of the rubber.

Vulcanization lowers the crystallinity as three-dimensional networks createobstacles in segments entering the crystal lattice. Lower crystallinity is alsoobserved in random copolymers.

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NR is obtained from the exudation of the rubber plant, Hevea brasiliensis. Therubber is obtained from the latex by coagulation, sheeting, drying, and baling.There are various internationally recognized market grades, common amongthem are ribbed smoked sheets and pale crepe. Natural rubber contains about90% rubber hydrocarbon as cis-1,4-polyisoprene along with naturally occurringresins, proteins, sugars, etc., that precipitate during latex coagulation. The av-erage molecular weight of polyisoprene in natural rubber ranges from 200,000to 500,000, with a relatively broad molecular weight distribution. As a result ofits broad molecular weight distribution, NR has excellent processing behavior.

cis-1,4-Polyisoprene unit






The α-methylene group of the polyisoprene units is reactive for vulcanizationwith sulfur. NR vulcanizates combine a range of properties that are of greattechnological interest. The individual property can be improved by the useof synthetic rubber, but a combination of high tensile strength, resilience, dy-namic properties, and good low temperature flexibility make NR indispensablefor several applications. The high tensile strength and tear resistance of NR vul-canizates is due to strain crystallization. Being nonpolar, NR swells in nonpolarsolvents. Reaction of the double bond in the polyisoprene unit with oxygen orozone results in degradation of the polymer. Styrene-Butadiene Rubber (SBR)

SBR is a copolymer of styrene and butadiene. The styrene content rangesfrom about 25 to 30 wt.%. The structure is given as



SBR is mainly prepared by emulsion polymerization, The monomers arerandomly arranged in the chain, and the butadiene part is mainly in the trans con-figuration (∼75%). Some 1,2-addition products are also formed. Depending on

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the temperature of polymerization, SBR may be classified into hot-polymerizedand cold-polymerized grades. The hot grades are highly branched compared tothe cold grades.

SBR can also be copolymerized in a solution process using alkyl lithiumcatalysts in a nonpolar solvent. These rubbers have much higher cis-1,4-butadiene content (50–55%), less chain branching, and narrower molecularweight distribution.

cis trans

1,4-Butadiene 1,2-Addition product







SBR does not crystallize even on stretching their vulcanizates, therefore,pure gum strength is generally low. It has better heat and aging resistance thanNR and is usually used in combination with NR and other rubbers. Isoprene-Isobutylene Rubber, Butyl Rubber (IIR)

Butyl rubber is a copolymer of 97 to 99.5 mole % of isobutylene and 0.5 to3 mole % isoprene. The isoprene unit provides the double bond required forsulfur vulcanization.


-(CH2 - C - )x(- CH2- C = CH-CH2)y-


IIR, Butyl rubber

It is produced by cationic polymerization in methylene chloride with AlCl3as catalyst, at subzero temperatures (−90◦C to −100◦C). The isobutylenemonomer units polymerize mainly in head-to-tail arrangements, and the iso-prene units in the polymer chains polymerize in trans 1,4-configuration. Themolecular weight ranges between 300,000 to 500,000. On halogenation of IIRin an inert organic solvent, a rapid electrophilic substitution takes place, andone halogen atom is substituted per isoprene unit, mainly in the allylic posi-tion. Thus, a small number of halogen atoms are incorporated into the polymerchain. These are known as chlorobutyl rubber (CIIR) or bromobutyl rubber(BIIR) depending on the halogen substituted. The polymer chains are highlysaturated and have a very regular structure due to the symmetrical nature of the

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monomer. As a result, butyl rubber exhibits very low gas permeability, ozone,heat, weathering, and chemical resistance. Butyl rubbers are self-reinforcingwith a high gum tensile strength. The halobutyl rubbers cure faster than butylrubber. Bromobutyl has much lower gas permeability and better resistance toaging, weathering, and heat than butyl rubber. Butyl and bromobutyl rubbersare especially used where low gas permeability is required. Ethylene Propylene Polymers (EPM) and Terpolymers (EPDM)

Copolymers of ethylene and propylene EPM are made by solution polymer-ization using vanadium containing alkyl aluminum, Ziegler-type catalyst. Theseare elastomers, but they do not contain any double bonds.

-( CH2-CH2)x-(CH-CH2)y-



For good elastomeric properties, the ethylene propylene ratio ranges from45–60 wt.%, and the monomers are arranged randomly. Consequently, thesepolymers are predominantly amorphous, and the pure gum strength is low.The molecular weight ranges between 200,000 to 300,000. Because EPM issaturated, the polymeric chain is cured by peroxides. The terpolymer EPDMcontains, in addition to the olefin monomers, a nonconjugated diene as thethird monomer, which renders EPDM able to be vulcanized by sulfur. Thecommon third monomers are dicyclopentadiene, ethylidene norbornene, and1,4-hexadiene. In these dienes, one double bond is capable of polymerizingwith the olefins, but the other is not a part of the main chain.

Dicyclopentadiene Ethylidene norbornene trans-1,4-Hexadiene

Both EPM and EPDM have excellent resistance to oxygen, ozone, heat, andUV radiation. Polychloroprene Rubber (CR)

These rubbers are produced by the emulsion polymerization process of2-chloro-1,3-butadiene. The polymer chains consists of approximately 98%

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1,4-addition products which are mostly trans in configuration, and the rest are1,2-addition products. The 1,2-addition product contains a chlorine atom at-tached to a tertiary allylic carbon atom that is highly activated and thus becomesthe curing site in the polymer chain.

trans-Polychloroprene unit 1,2-Addition product





CH2~~~C ~~~H2 C




The temperature of polymerization has an important bearing on the polymerstructure. At higher temperatures, there is less uniformity in the chain due tolarge proportions of 1,2 and 3,4 moieties and other isomers in the monomericsequences. On the other hand, at lower temperatures, the polymeric chain ismore regular. The CR is also available in sulfur-modified grades. The polymerchains have Sx groups, and this aids processing due to easy depolymerization.Unlike the diene rubbers, CRs are not vulcanized by sulfur but are vulcanizedby metal oxides—a combination of MgO and ZnO.

Polychloroprene stiffens at low temperatures. This is due to second-ordertransition and crystallization. The rate of crystallization is most rapid at −10◦C.Though the stiffening is reversible, it is detrimental for the production of certaingoods. Incorporation of low temperature plasticizers like butyloleate can lowerthe stiffening temperature of CR compounds. CR produced by high temperaturepolymerization has a much lower rate of crystallization than that producedat a low temperature. The high crystallizable grades are useful as adhesives.For production of coated fabrics, materials with long crystallization times arechosen, as softness and flexibility are more important than ability to withstandheavy stress. The gum vulcanizates of CR show high tensile strength becauseof strain crystallization, but the resilience is lower. Polychloroprene rubbersare resistant to oxidation, ozone degradation, and flex cracking, and, becauseof the chlorine atom in the molecule, are inherently flame resistant. Becauseof its polar nature, the rubber is resistant to hydrocarbons, fats, oils, and mostchemicals. It is used for applications requiring weather, oil, ozone, and flameresistance. Nitrile Rubber (NBR)

NBR is a copolymer of acrylonitrile and butadiene obtained by emulsionpolymerization. The acrylonitrile content varies from 18–50%, depending on

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the properties desired. The molecular weight ranges from 20,000 to 100,000.

-(-CH2 - CH = CH - CH2 )x- ( CH2 - CH )y--CN


Like SBR, varying temperatures of polymerization produce differentgrades of NBR. NBR produced at low temperatures shows less branchingthan hot rubbers. The steric configuration, i.e., cis-1,4, trans-1,4, and trans-1,2structures are also influenced by polymerization temperature. The lack of com-positional uniformity along the polymer chains prevents formation of crysta-llites on extension. This results in poor tensile properties of NBR gumvulcanizates.

Nitrile rubbers are of special interest because of their high degree of resistanceto fuels, oils, and fats. An increase in acrylonitrile (AN) content increases its oilresistance because of enhancement of polarity of the rubber. NBR has a low gaspermeability. Increase of AN percentage in NBR lowers its gas permeabilitybut adversely affects its low temperature flexibility and resilience. NBR isextensively used where oil resistance is required. Chlorosulfonated Polyethylene Rubber (CSM)

CSM is produced by reaction of polyethylene solution with chlorine andsulfur dioxide in the presence of UV radiation.

--CH2 - CH - CH 2 - CH 2 -CH2- CH---

Cl SO2ClCSM unit

Commercial grades contain 25–40 wt.% of chlorine and about 1% of S. Thechlorine and sulfur are randomly distributed along the polymer chain. It iscross-linked by metal oxides through chlorine atom and chlorosulfonyl group.These rubbers are characterized by a unique combination of special propertieslike ozone resistance, flame retardance and resistance to corrosive chemicalsand oxidizing agents. Silicone Rubber

Silicone rubber is obtained from silicones. Chemically, silicones are poly-siloxanes containing Si-O- bonds. The most important polymers are poly-dimethyl siloxane, polymethyl phenyl siloxane, and vinyl methyl siloxane. They

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are manufactured by the hydrolysis of the appropriate dichlorosilane R2SiCl2.Silicones are available in a wide range of molecular weights and viscosities,from fluids to gums.


-O-Si--O- -O--Si--O- --O--Si--O--



Dimethyl siloxane unit Methyl phenyl siloxane unit Vinyl methyl siloxane unit

There are three major ways of curing silicone rubbers, viz., peroxide cure,hydrosilation, and condensation cure [5,6].

(1) Peroxide initiated cure: ROOR�→ 2RO


--O-Si-CH =CH2 + CH3-Si-O- → --O-Si--CH2-CH2-CH-Si-O- -→ cross-link




This is a one-part system where the peroxide is activated on heating above100◦C. The vinyl group facilitates free radical reaction with the formationof vinyl to methyl and methyl to methyl bonds.

(2) Hydrosilation: an addition reaction occurs between vinyl siloxane and silox-ane containing Si-H group catalyzed by Pt (chloroplatinic acid). The cross-link occurs due to multiple functionality of both reactants. This is a two-partreaction with one part containing vinyl siloxane with Pt catalyst and theother containing silicone with Si-H functionality. The two liquid parts per-mit direct processing including coating without solvent and are known asliquid silicone rubber LSR.

CH3 CH3 Pt CH3 CH3

-O-Si-CH=CH2 + H-Si -O- -----→ -O-Si-CH2--CH2-Si-O- --→ cross link


PDMS with Si-H group


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(3) Condensation cure: condensation reaction between siloxanes with terminalhydroxyl groups, -silanols, and a cross-linker,–tri or tetra functional organosilicon compound, leads to cross-linking/cure.

HO-Si-(O -Si-O)n-Si-OH + X-Si-X → -Si-(O-Si-O-)n Si-O --→ cross-link


Silanol Cross-linker




Common cross-linking agents are tetraethyl ortho silicate (X = OEt), triace-toxy silane (X = OAc ), etc. Condensation occurs at room temperature in thepresence of metal soap catalysts, typically, stannous octoate and moisture. Therubbers are known as room temperature vulcanizates (RTV). The RTVs may betwo-part systems or one-part systems. In a one-part system, the common cross-linker is methyl triacetoxy silane, and the reactants are stored in anhydrouscondition.

The molecular weights of hot vulcanizates range from 300,000 to 1,000,000.The Si-O bond energy is higher than that of the C-C bond, as such, the poly-siloxane chain is thermally and oxidatively much more stable than organichydrocarbon chains. Silicone rubbers can also retain their flexibility to as lowas −100◦C. The stability of silicones over a wide range of temperatures is out-standing and is not found in any other rubber. Silicones are extraordinarily re-sistant to aging, weathering, and ozone. They have, however, lower mechanicalproperties, but they do not change much with temperature. The vulcanizates arehydrophobic and are resistant to chemicals. They form transparent/translucentcoatings. Because of their unique properties, they find specialized applications,for example, gaskets, O-rings, wire cable, etc.


Various chemicals, fillers, accelerators, and cross-linking agents are addedto rubber to facilitate processing and vulcanization. The properties of the endproduct can be significantly altered by proper choice and amount of the com-pounding ingredients to meet the diverse end-use requirements.

Typical ingredients of a rubber formulation are as follows:

� raw rubber� cross-linking agents� accelerators� accelerator activators

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� antidegradants� fillers, reinforcing, diluents� processing aids� pigments and dyes� special additives, e.g., flame retardants, fungicides

The cross-linking agent for natural rubber is sulfur along with organic ac-celerators. Zinc oxide and stearic acid are used as activators of the accelerator.The synthetic olefin rubbers like SBR, NBR, butyl, and EPDM can also becured by sulfur system. For butyl and EPDM rubbers having low unsaturation,more active accelerators are used with a higher temperature of cure. Apart fromelemental sulfur, organic compunds that liberate sulfur at the temperature ofvulcanization can also be used for vulcanization, e.g., dithiodimorpholine and2-morpholine-dithio benzothiazole.

The nonolefin rubbers, polychloroprene and chlorosulfonated polyethylenesare cured by metal oxides. In the case of polychloroprene, the curing is doneby zinc oxide and magnesium oxide, because sulfur cure is not possible asthe double bond is hindered by the neighboring chlorine atom. The zinc oxidereacts with chlorine atoms of the 1,2 units present in the chain after an allylicshift. Magnesia also acts as scavenger of the chlorine atom. More rapid cure isachieved by the use of organic accelerators like ethylene thiourea. Chlorosul-fonated polyethylene is cured by litharge and magnesia, along with accelera-tors. The curing occurs by ionic and covalent bond formation through sulfonylchloride groups.

In the case of ethylene propylene rubber (EPM) and silicones, peroxidesare the cross-linking agents. In EPM, the tertiary carbon atom at the point ofbranching is attacked by peroxides with H abstraction, generation of a ter-tiary free radical, followed by termination, forming a cross-link. The cross-linking of polydimethyl siloxane by peroxides, as discussed earlier, occursthrough abstraction of H from the methyl group with the formation of free rad-ical followed by termination. More rapid vulcanization is achieved when vinylgroups are present in the chain. Some common peroxides are benzoyl peroxide,2,4-dichlorobenzoyl peroxide, dicumyl peroxide, etc.

The accelerators used for sulfur vulcanization of olefin rubbers are of var-ious types depending on the rate of cure, i.e., medium, semi-ultra, and ultraaccelerators. The chemical types are aldehyde amine, guanidines, thiazoles,sulfenamides, dithiocarbamates, thiuram sulfides, and xanthates. The actionof the accelerator is further enhanced by activators. Zinc oxide and stearicacid system for sulfur vulcanized rubbers is most common, but other zincsalts of fatty acids like zinc laurate can be used. Most of the rubber formu-lations contain antidegradants, antioxidants, and antiozonants. They functionby either capturing the free radical formed during the degradation process orby decomposing the peroxides and hydroperoxides produced into nonreactive

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fragments. The majority of the commercially available inhibitors belong totwo main chemical classes: amines and phenolics. Fillers are incorporatedinto a rubber formulation for reinforcement, i.e., for enhancement of ten-sile strength, abrasion resistance, and tear resistance, or as diluent to reducecost. The most common reinforcing filler is carbon black. Among the non-black reinforcing fillers, common are precipitated silica, fume silica, calciumsilicate, hydrated aluminium silicate (clay), etc. Barytes, whitings, talc, chalk,kaolin, and kieselguhr are some of the other fillers used. Plasticizers andsofteners are added to rubber compounds to aid various processing opera-tions of mixing, calendering, extruding, and molding. These include a widerange of chemicals such as petroleum products, oils, jelly, wax, coal tar prod-ucts, pine tar, fatty acid salts, factice (reaction product of vegetable oil andsulfur), and esters of organic acids. Peptizers are also added to increase theefficiency of molecular breakdown, facilitating the mastication process.Pentachlorophenol, its zinc salt, and di-(o-benzamido phenyl) disulfide, arecommon peptizing agents. For noncarbon black rubber compounds, coloringmaterials are used, which are generally colored inorganic compounds. Some-times, special purpose additives are added to obtain specific properties, likeblowing agents for cellular rubber, flame retardants, like chlorinated paraffins,zinc borate, etc.


For coating of rubber on fabric, it has to be properly processed. The stepsinvolved are as follows:

� mastication or milling� compounding� coating by calendering� preparing dough and spread coating� vulcanizing the coated fabric

The processing and machinery required for rubber are different from thoserequired for coating other polymers. A brief account of the processing steps arediscussed here. Mastication and Compounding

Raw rubber is masticated to decrease its viscosity to a desired level forincorporation of the compounding ingredients and their proper dispersion. Aproper adjustment of viscosity is also required for various processing operations.When rubber has all of the ingredients needed, it is known as a “compound.” Ifsome ingredients have been deliberately withheld, particularly curing agents,the partially completed compound is known as the master batch. Mastication isdone generally in mixing mills or in internal mixers.

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Figure 1.1 Line diagram of a two-roll mixing mill. Courtesy M/s Slach Hydratecs Equipment,New Delhi, India.

Mixing mills are used for small-size compounds and as a follow-up to inter-nal mixers. They consist of two horizontal and parallel rolls made from hardcastings, that are supported through strong bearings in the mill frame. The dis-tance between the rolls is adjustable. The two rolls move in opposite directionsat different speeds, the back roll running faster than the front (Figure 1.1). Theratio of the two speeds is known as the friction ratio, which is about 1:1.25 fornatural rubber. Frictioning promotes tearing, kneading, and mixing of the rub-ber mass and the ingredients in the roll nip. Side guides are provided to preventrubber from flowing to the bearings. The rolls can be cooled or heated withwater or steam by circulating water through a drilled core or through peripheralholes. Raw rubber is first placed between the rolls, the elastomer is torn, andthen it wraps around the front roll. After several passages, a continuous band isformed. The degree of mastication is controlled by the temperature of the roll,the size of the nip, and the number of passes. The compounds are then added ina well-defined sequence, such as accelerators, antioxidants, factice, pigments,fillers, and sulfur. If peptizers are used, they are added first. At the end, thecompounded layer is cut off repeatedly in order to homogenize it.

The mixing in an internal mixer is done in a closed chamber by rotatingkneading rolls. The mixer can handle larger batches. The internal mixer consistsof a mixing chamber shaped in the form of a horizontal figure of eight. Tworotors are fitted into the chamber that rotate at different speeds to maintain ahigh friction ratio. The walls of the mixing chamber and the rotors are equippedwith cavities for cooling or heating. The mixing chamber has a filling device atthe top, which is used for adding ingredients.

The chamber is closed by a pneumatically operated ram to ensure that the rub-ber and ingredients are in proper contact. A ram pressure of about 2–12 kg/cm2

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Figure 1.2 Line diagram of a drop door type internal mixer. Courtesy M/s Slach Hydratecs Equip-ment, New Delhi, India.

is applied for this purpose. Higher ram thrust results in better mixing efficiency.The design of the rotors may be (1) tangential type, where the kneading actionis between the rotors and the jacket or (2) intermeshing type, where the knead-ing occurs between the rotors. A drop door type internal mixer is shown inFigure 1.2.

The rotor speed varies between 20–60 rpm. The capacity of the internal mixercan vary widely, but mixers of 200 kg batch size are quite popular. The operationof the mixer consists of adding rubber in split or pelletized forms. After a

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short mastication, the compounding ingredients are added. The incorporationof rubber and other ingredients can be partly or fully automated. Normally,complete mixing is not done in an internal mixer because of the scorch problem.Master batch is initially prepared and the remaining ingredients are added to itafterwards in a mixing mill. The compound coming out of the internal mixer isin the form of lumps and has to be cooled and homogenized in sheeting mills.Mixing can be done continuously in a single- or twin-screw extruder, however,extruders are not very popular for mixing rubbers. Spreading Dough

Preparation of dough of the right consistency is of great importance for spreadcoating. The compounded rubber is cut thinly in narrow strips or pieces andsoaked in a proper solvent for a few hours. Organic solvents capable of dis-solving the elastomers are selected for the purpose, keeping the cost factor inview. Mixed solvents are also used quite often for better solvating properties,enabling preparation of a more concentrated solution. Toluene, aromatic andchlorinated hydrocarbons, and esters are common solvents used. For naturalrubber, toluene is usually used. The soaked mass is then transferred into solu-tion kneaders for preparation of a homogenous dough. The solution kneadersconsist of a semicircular trough with a lid and two kneading paddles in the shapeof Z or sigma. The paddles turn with a differential speed, the forward one is 1.5to 2 times faster than the back. The trough is double walled to permit cooling andis closed by a movable lid. The rotating paddles disintegrate the swollen masswhile continuously wetting the rubber. The agitation is continued until a doughof the right consistency is produced; this may take up to 12 hrs. The doughis emptied from the trough either by tilting or by discharge screws as per thedesign of the kneader. However, high speed Ross mixers do this job in 30 minto 1 h. Vulcanization of Rubberized Fabrics

Vulcanization can be carried out as a batch process in a steam autoclave usingsaturated steam. An autoclave is a cylindrical pressure vessel, normally usedin the horizontal position. Curing can also be done by hot air under pressure.However, because the heat transfer coefficient of air is lower than steam, aircuring requires higher curing time, needing a change in formulation. Moreover,the oxygen of air can oxidize the elastomer. In steam curing, on the other hand,formation of condensed water can lead to unsightly water spots, and local under-curing. This problem is solved by coating the uncured article with a wettingagent.

Continuous drum cure, also known as rotocure, is also used. In this process,the curing of the coated fabric is achieved by placing the fabric in contact witha rotating steam-heated drum (Figure 1.3).

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Figure 1.3 Rotocure process: (1) steel drum, (2) steel belt, (3) tension roll, and (4) fabric.

A steel band runs over about two-thirds of the circumference of a slowlyrotating steam-heated steel drum. The steel band is pressed against the drum.The sheet to be vulcanized runs between the drum and the steel band and ispressed firmly against the drum with a pressure of about 5–6 kg/cm2. The sheetslowly moves with the rotation of the drum. Vulcanization occurs because ofthe temperature of the drum and the pressure on the sheet created by the steelband.



Polyvinyl chloride (PVC) is one of the few synthetic polymers that has foundwide industrial application. The popularity of PVC is due its low cost, excellentphysical properties, unique ability to be compounded with additives, and use-fulness for a wide range of applications and processability by a wide variety oftechniques. The repeat unit of PVC is



The units are linked mainly head to tail, with very few head to head links.PVC is considered to be an amorphous polymer. The crystallinity is only about10%. This is attributed to the nonregular position of the chlorine atoms aroundthe carbon chain. Branching is low in PVC. Lower polymerization temperaturefavors more linear structure. The molecular weight (Mn ) of commercial resins,ranges from 50,000 to 100,000.

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It is produced by the addition polymerization of vinyl chloride, CH2=CH-Cl.The methods of polymerization are suspension, emulsion, mass, and solutionprocesses. Among these, suspension is the most favored commercial process,contributing to about 80% of total polymer production. The emulsion and massprocesses contribute to about 10% each. The resins obtained from suspensionand mass polymerization are porous and absorb plasticizer rapidly. Polymerobtained by the emulsion process is nonporous, with very fine particle size. Itquickly and reversibly absorbs plasticizer once heated at temperatures above80◦C. The main use of emulsion PVC is in plastisol and organosol preparations,which are extensively used in coating and slush molding. The solution processis almost exclusively restricted to the manufacture of PVC copolymers for usein surface coatings.


The physical forms of PVC resins are diverse which permits a wide rangeof processing techniques. The resins are classified on the basis of importantproperties required for processing. The specifications of the resins have beenstandardized by ASTM and ISO; and a nomenclature system to designate theproperties evolved. The characteristics of general purpose resins (G) and dis-persion resins (D) are different.

The important characteristics of general purpose resins are as follows:

(1) Molecular weight: in industrial practice, dilute solution viscosity is nor-mally determined as an index of molecular weight. The results are com-monly expressed in terms of K value or viscosity number. The Mv can bedetermined from the Mark-Houwink equation. A high K value denotes highmolecular weight, a high melt viscosity of the unplasticized PVC requiringhigher processing temperature.

(2) Particle size: particle size and particle size distribution influence compound-ing and processing properties.

(3) Bulk density

(4) Dry flow: this property is a measure of the ease of handling of granularresins.

(5) Plasticizer absorption: it is a measure of the capacity of the resin to absorbplasticizer, yet remain a free-flowing powder. It is dependent on surfaceproperties of the resin powder.

(6) Electrical conductivity: this test is intended to distinguish between electricaland nonelectrical grades.

Important characteristics for dispersion resins are as follows:

(1) Molecular weight

(2) Particle size

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(3) Settling

(4) Plastisol viscosity: usually determined by viscometer at specified shearrates with a specified concentration of plasticizer (dioctyl phathalate)

(5) Plastisol fusion: it denotes the complete solvation of the resin by the plas-ticizer and is a function of solvating power of plasticizer, temperature, andtime. Determination of clear point and measurement of rate of increase ofviscosity at a constant temperature are some of the methods of determina-tion of fusion.


For processing and imparting properties for special applications, PVC iscompounded with a variety of additives. Some of the important additives are asfollows:

� plasticizers� heat stabilizers� fillers� lubricants� colorants� flame retardants Plasticizers

Plasticizers are an important additive of PVC resin, because the majority ofPVC products are plasticized. These are liquids of low or negligible volatil-ity or low molecular weight solids, which when incorporated into the poly-mer, improve its processability and impart end product softness, flexibility, andextensibility. The other concomitant effects of plasticization are lowering ofTg and softening temperature, reduction of strength, and increased impact re-sistance. The plasticizer acts by lowering the intermolecular forces betweenthe polymer chains. The plasticizer should be compatible with the polymer, orexudation will occur. Those plasticizers that are highly compatible with PVCare known as primary plasticizers, while plasticizers that have limited compa-tibility are known as secondary plasticizers. Secondary plasticizers are addedto impart special properties or to reduce cost. Plasticizer Characteristics

The main parameters for ascertaining the effectiveness of a plasticizer arecompatibility, efficiency, and permanence.

(1) Compatibility: this can be determined from solubility parameter δ and FloryHuggins interaction parameter χ . The PVC-plasticizer system is considered

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compatible if its solubility parameters are nearly equal. Again, if the χ valueis low (<0.3), the system is considered compatible. Compatibility can alsobe determined from clear point, which is the temperature at which the PVC-plasticizer mixture becomes clear. The lower the clear point temperature,the greater the compatibility.

(2) Efficiency: technologically, it is the amount of plasticizer required to pro-duce a selected property of practical interest, like hardness, flexibility, ormodulus. The efficiency of plasticizer can also be gauged by the loweringof Tg , and changes in dynamic mechanical properties.

(3) Permanence: the plasticizer may be lost from the compounded resin byvaporization into the atmosphere, extraction in contact with a liquid, ormigration into a solid in intimate contact with the plasticized PVC. Perma-nence can be determined by weight loss measurements on exposure to theextraction media. Plasticizer Types

(1) Phthalates: these are the largest and most widely used group of plasti-cizers. The esterifying alchohol ranges from methyl to tridecyl. The lowerchain length esters have high solvating power but suffer from high volatilityand poor low temperature properties. Medium chain C8 phthalates possessoptimum properties. The longer chain C10–C13 esters have reduced solvat-ing power and efficiency, though low volatility. Di-2-ethylhexyl phthalate(DOP) and diisoctyl phthalate (DIOP) are extensively used in industry be-cause of their better balance of properties.

(2) Phosphates: these are organic esters of phosphoric acids. The triaryl phos-phates, like tricresyl (TCP) and trixylyl (TXP), are by far the most im-portant phosphate plasticizers.The triaryl phosphates offer excellent flameretardance, good solvating power, and good compatibility, but poorer lowtemperature properties.

(3) Aliphatic diesters: in this category are esters of adipic, azelaic, and sebacicacids of branched chain alcohols such as isooctanol, 2-ethylhexanol, orisodecanol. These impart low temperature flexibility to PVC compositions.Their compatibility is, however, low, and they are categorized as secondaryplasticizers.

(4) Epoxies: epoxidized soybean oil and linseed oil exhibit good plasticizingand stabilizing actions. They possess low volatility and good resistance toextraction.

(5) Polymeric plasticizers: the majority of commercial plasticizers of this classare saturated polyesters, synthesized by the reaction of a diol and dicar-boxylic acid along with an end capping agent, which may be a monohydricalcohol or monocarboxylic acid. An increase in molecular weight results

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in improved permanence and lower volatility, but it adversely affects lowtemperature properties and compatibilty. Heat Stabilizers

Unless suitably protected, PVC undergoes degradation at the processingtemperatures. The manifestations of the degradation include the evolution ofhydrogen chloride, development of color from light yellow to reddish brown,and deterioration of mechanical properties. The degradation occurs due tothe progressive dehydrochlorination of the polymer chains with the forma-tion of conjugated double-bond polyenes, possibly by a free radical mech-anism. The site of initiation could be a chlorine atom attached to a tertiarycarbon atom at the site of branching. As shown in Figure 1.4, the first step isthe formation of an allylic group, whose Cl atom is strongly activated by theneighboring double bond favoring further elimination of HCl. The HCl acts as anautocatalyst.

In addition to the polyene formation, the polymer undergoes chain scission,oxidation, cross-linking, and some cyclization. The Diels Alder reaction be-tween polyene moieties of neighboring chains is believed to be responsible formuch of the cross-linking. PVC also undergoes photodegradation on exposureto light in the presence as well as in the absence of oxygen. The manifesta-tions and degradation products are similar to those of thermal degradation. Inplasticized PVC, exudation of plasticizers from PVC occurs on weathering,which has been attributed to their partial exclusion from the areas where cross-linking has occurred. A heat stabilizer should prevent the reaction responsiblefor degradation of PVC. It should bind the liberated hydrogen chloride, deac-tivate potential initiation sites by substituting stable groups for labile chlorine,disrupt formation of polyene sequence, and deactivate the free radicals. Mor-ever, the stabilizer should be compatible with the polymer and other additivesin the compound.

The heat stabilizers may be classified as lead compounds, organo tin com-pounds, compounds of other metals like barium, cadmium, and zinc, and organic

Figure 1.4 Formation of polyene structure.

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compounds. Common lead and tin compounds are lead phosphate, dibasic leadstearate, dibutyl tin laurate, dibutyl tin maleate, etc. Barium, cadmium, andzinc salts, when used in combination, impart excellent stability and show syn-ergistic effect. The compounds are salts of fatty acids, like laurates, stearates,and octoates. These compounds are used in calendered goods, plastisols, floor-ings, and coated fabrics. Organic stabilizers are epoxidized oils, phosphites,and polyhydric alcohols. They are normally regarded as secondary stabilizersin conjunction with Ba-Cd-Zn stabilizers.

For some special applications antioxidants are added to the PVC compound.They are usually phenolics like 2,6-di-t-butyl-4-methyl phenol and 3-(3,5-di-t-butyl-4-hydroxy phenyl) octadecyl propionate. The degradation of PVC by UVradiation can be prevented by incorporation of UV absorbers like derivativesof 2-hydroxy bezophenone, benzo triazoles, etc., or by addition of inorganicparticulate screening agents like carbon black and titanium dioxide. Other Additives

(1) Fillers: the primary role of a filler in PVC is to reducte cost, but they canplay a functional role by improving processing and properties of the endproduct. The common fillers are (1) calcium carbonate fillers—whiting,and marble dust, (2) silicates—clay, talc, and asbestos, and (3) barytes.

(2) Lubricants: the role of a lubricant is to facilitate processing and controlthe processing rate. Mineral oil, silicone oils, vegetable oils, and waxes arecommon lubricants. Metal stearates of Pb, Ba, Cd, and Ca may be usedfor the dual purpose of stabilizing and lubricating. The compatibility oflubricants is low, resulting in their exudation at processing conditions.

(3) Colorants: the colorants of PVC are inorganic and organic pigments. Theinorganic pigments include titanium dioxide, chromium oxide, ultramarineblue, molybdate orange, etc. The organic pigments are phthalocyanines,quinacridines, and benzidines. The inorganic pigments have excellent heatresistance, light stability, and opacity.

(4) Flame retardants: the inherent flame retardant property of PVC due to thepresence of a chlorine atom is affected by the addition of flammable plasti-cizers. Antimony trioxide and borates of zinc and barium are widely usedfor this purpose. Chlorinated paraffins and phosphate ester-plasticizers alsoact as flame retardants.


These are fluids in which fine PVC particles are dispersed in plasticizers.Plastisol or pastes (used synonymously) do not contain any solvent/volatilecomponents. An organosol is a plastisol containing volatile organic solvents.

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The viscosity of plastisols varies from pourable liquids to heavy pastes. PVCpastes have two important characteristics.

(1) They are liquids and can be processed in that condition. The processingconditions are determined by the property of the paste at ambient temper-ature.

(2) On application of heat, when required, they fuse to viscous solutions ofpolymer in plasticizer, and on cooling, they result in familiar plasticizedPVC.

A typical formulation consists of resin, plasticizer, stabilizer, fillers, pigments,and viscosity modifiers. Unlike solid formulations, lubricants and polymericmodifiers are not added in pastes. The natures and roles of various ingredientsare discussed below. Resins

The requirements of paste polymers are rather conflicting. They should havethe following:

a. Resistance at room temperature to the plasticizer for stability

b. Good affinity for plasticizer to rapidly dissolve in it at an appropriate tem-perature for proper gelation and fusion

Resins made by mass or suspension are porous granules and will absorba high level of plasticizer to form a sticky agglomerate. They are not usedfor making plastisols. Paste resins are made by emulsion or microsuspensionpolymerization and are finished by spray-drying techniques. They have highsphericity and a fairly dense surface, so that penetration of plasticizer at roomtemperature is low. Particle size ranges from 0.1 µm to 3 µm. Particle sizeand particle size distribution profoundly influence the viscosity of the paste.Extender resins of particle size 80–140 µm having a nonabsorbent surfaceare generally added to lower the viscosity of the paste. Because emulsion-grade resins are expensive, incorporation of extender resins lowers the cost.Paste resins are usually homopolymers, but copolymers with vinyl acetate(3–10%) are used to lower the fusion temperature. Although paste resins areresistant to swelling or solution by plasticizer at ambient temperature, slowsolvation still occurs, otherwise settling of the resin will take place. The slowsolvation results in an increase in viscosity of the paste on storage, known asaging. Higher molecular weight resins give products with superior physicalproperties, but the fusion temperature is increased.

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The important factors in selecting a plasticizer are viscosity, viscosity stability,clarity, compatibility, permanence, and fusion temperature. A plasticizer shouldhave adequate compatibility for fusion of the paste at an elevated temperature.Alkyl phthalates are the most common primary plasticizer. Stabilizers

It is preferable to use liquid stabilizers for good dispersion. Care should betaken that the stabilizer is compatible with other liquids of the paste or elseprecipitation may occur. Ba-Zn and Ca-Zn combinations are generally used.Tin-based systems are used where clarity is desired. Fillers

Various fillers like clay, calcium carbonate, barytes, etc., are added to thepastes. They affect the flow properties and aging characteristics of the paste.Fillers increase the paste viscosity due to an increase of the particulate phase andadsorption of plasticizer by the filler particles. The adsorption can be reducedby using coated fillers, such as with organic titanates. Viscosity Depressants

These additives, which are surface-active agents, lower the viscosity andimprove viscosity stability and air release properties. Polyethylene glycol deri-vatives are generally effective. Thickeners

For certain applications, paste should have a high viscosity at low shear ratesand a low viscosity at high shear rates. An example is in spray coating or dipcoating where no sag/drip property is desired. Various thickening agents likefumed silica, special bentonites, and aluminium stearates are used. These forma gel structure, and the paste varies in consistency from butter to putty. Theyare also known as plastigels. Blowing Agents

The addition of azo dicarbonamide, which decomposes to form nitrogen gas,is the common method used to produce expanded vinyl. The decomposition ofthe blowing agent should occur at or above the fusion temperature for the for-mation of a closed-cell structure. If the blowing agent decomposes completely

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before gelation, an open-cell structure will be formed. The cell size is deter-mined by the rate of decomposition of the blowing agent and the melt viscosityof the fused composition. Low molecular weight polymers are used in foamformulation, so that the melt viscosity of the fused paste is low. Foam pastes arenot commonly deaerated prior to use as the air present acts as nucleating agentfor cell formation as the azo decomposes. Blow ratios are controlled by thequantity of azo compound. A high blow ratio will blow the foam apart duringfusion. Manufacture

The pastes are made in a simple paddle-type mixer that provides an interme-diate level of shear. The temperature should not rise during mixing. The mixingis generally carried out in vacuum, or entrapped air is removed after mixing bysubsequent deaeration. The presence of air may result in bubbles and loss ofclarity of the end product. Fusion

On heating, the liquid paste is converted into a solid. As the temperatureof the paste rises, more plasticizer penetrates the polymer particles, causingthem to swell. This process continues until at about 100 ◦C, the liquid phasedisappears completely with the formation of a gel (gelation temperature). Onfurther heating, a solution of polymer and plasticizer is formed, with the forma-tion of an homogenous plasticized PVC melt (fusion temperature). On cooling,solid plasticized PVC is obtained. The processes of gelation and fusion are theconversion of suspended polymer particles in a plasticizer to a solid containingdispersion of plasticizer in a continuous polymer matrix. This is, therefore, aphase inversion (Figure 1.5). Organosols

Diluents are added to reduce the viscosity of plastisols, to make it suitablefor spray, roller, brush, and other forms of coatings. The thinned plastisols areknown as organosols. The diluents are nonsolvents of PVC, like toluene, xylene,

Figure 1.5 Gelation and fusion processes.

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naphtha, and mineral spirits. Addition of these diluents shifts the solubilityparameter of the dispersing medium away from PVC, lowering the solvationand reducing the viscosity. As the diluent level increases, the viscosity passesthrough a minima and then increases with further dilution. This increase ofviscosity is due to flocculation of the plastisol resin. Uses in Coating

Pastes are extensively used for flexible coatings applied by dipping, spray-ing, or the spreading process. The products are diverse, including upholstery,luggage fabric, wall coverings, floor coverings, tarpaulins, and shoe uppers.


The purpose of compounding is to blend the resin and additives into a ho-mogeneous, well-dispersed form appropriate for further processing. PVC for-mulations are used in the industry in liquid phase, i.e., paste and solutions, orin solid form, as powder or pellets. Solid phase compounding can be broadlycategorized into two types, melt compounding and dry blending. Melt Compounding

In this process, a premix is made in a ribbon or tumble blender. The pre-mix is then fluxed and pelletized or may be directly sent to high shear mixersthat break down the resin and simultaneously disperse and blend the additiveswith the fluxed resin. Two types of mixers are widely used: the batch-typemixers offer greater flexibility when frequent product and formulation changesare encountered and the continuous mixers are used when large volume andsteady throughput is required. The batch-type mixers can be either a tworoll mill or an internal mixer of the Banbury type, similar to thoseused for rubber compounding. Continuous mixers are single- or double-screwextruders. Dry Blending

Dry blending is a process of adding liquid and dry compounding ingredi-ents into PVC to produce a granular, free-flowing powder. The resin is notfused during the compounding operation. The individual particles of the dryblend are much like the initial resin. Their size is, however, greater due to ab-sorption of plasticizer and other additives. The resins used for commerciallyplasticized applications are suspension and mass polymerized homopolymerof PVC. The resins are porous and have high surface area for rapid absorptionof plasticizer, etc. It is desirable to select a resin with a narrow particle size

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range. The penetration of plasticizer in a large particle size causes “fish eyes”in the processed product. Fine particles tend to float in the molten mass causingsurface imperfections in the product.

The dry blends are produced in different mixing equipment. Mainly, two typesof mixers are used: the steam jacketed ribbon blender or the high-intensitybatch mixer. They are coupled to a cooling blender. In a ribbon blender, theresin and dry ingredients are added first and allowed to mix for a short time tobreak the agglomerates. The blender may be heated, if required, to increase theabsorptivity of the resin for the plasticizer. The heated plasticizer mix containingother liquid additives is then sprayed onto the resin mix, and the blender isheated. The time and temperature of the mix is dependent on the formulation, butnormally the temperature is between 100◦C to 130◦C, and the time is between10–20 min. The powder is next discharged to a coupled cooling mixer. In ahigh-intensity mixer, the procedure of adding the ingredients is similar. In thesemixers, heating is mainly due to the mechanical energy of the mixing process,i.e., shear and friction.

Sintered dry blends are partly fluxed pellets that are obtained by heating thedry blend to near its fusion point to sinter the particles into agglomerates. Thesecan vary in size from coarse powder to regular pellets. These sintered blendsare nondusting, easy to transport, more homogenous than dry blends, and havebetter processability. The production is usually done in a high-intensity mixercoupled with a cooler mixer. After the dry blend is formed, the temperature ofthe blender is increased until the particles agglomerate to the desired size. Theblend is then discharged rapidly into the cooling blender, where it is rapidlycooled, and further agglomeration is prevented.

The investment in equipment for dry blending is substantially low, enablingprocessors to carry out their own blending.



Polyurethanes [12–15] are polyaddition products of di- or polyisocyanatewith a di- or polyfunctional alcohol (polyol).

n ( NCO-R-NCO) + n ( HO-R�-OH ) →

OCN-( -R-NH-C-O-R�-O-C-NH-)n-1 -R-NH-C-O-R�-OH || || ||O O O

Urethane (4)

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If the functionalities of the reactants are three or more, branched or cross-linked polymers are formed. Variations in the R and R′ segments of the polyad-dition reaction shown above permit preparation of polyurethane to meet specificneeds. The extent of cross-linking, chain flexibility, and intermolecular forcescan be varied almost independently. The range of polyurethane products is thusquite diverse and includes fibers, soft and hard elastomers, and flexible andrigid foams.

The two important building blocks are isocyanates and polyols. Chainextenders like short-chain diols or diamines and catalysts are frequently usedin the synthesis of the polymer. A brief account of the chemistry of theraw materials and polyurethanes is being discussed to obtain a properperspective.

1.3.2 BUILDING BLOCKS OF POLYURETHANE The Isocyanates Basic Reactions of Isocyanates

The isocyanate group is highly reactive. Its most important reaction is thenucleophilic addition reaction of compounds containing an active hydrogenatom. The general equation is given by

R-N= C = O + HX → R -N -C -X H O |||


Some important reactions are given below:

(1) Reactions with compound containing -OH groups

O ||

R - N =C = O + R� OH → R -NH -C -O R�

Urethane group (6)

The trivial name urethane, which is used for ethyl carbamate, is used as thegeneric name for all polyurethanes. Primary, secondary, and tertiary -OHshow a decreasing order of reactivity.

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(2) Reactions with compound containing -NH groups

O ||

R - N = C = O + R� NH2 → R -NH -C- NHR�

Substituted urea (7)

Primary and secondary amines react vigorously forming substituted urea.Primary amines react faster than the secondary amines. Ammonia and hy-drazines react similarly.

(3) The urethanes and ureas formed by the above reactions [Equations (6)and (7)] still possess acidic protons and react further with additional iso-cyanates to form allophanates [Equation (8)] and biurets [Equation (9)],respectively.

O O || ||

R - NH -C - O R� + R - N=C=O → R-N - C -OR�

O = C -NH - R

Allophanate (8)

O O || ||

R - NH - C-NH-R� + R - N =C =O → R - N - C -- NH-R�

O= C-NH-RBiuret (9)

In the case of polyisocyanates, the above reactions [Equations (8) and (9)]lead to the formation of branching in the polymer.

(4) Reaction with water: water reacts with isocyanate to form unstable car-bamic acid that splits into carbon dioxide and the corresponding amine.The amine immediately reacts again with another molecule of isocyanateto form symmetrical urea. The carbon dioxide acts as a blowing agent inthe production of foams.

O RNCO O || ||

R - N =C = O + H2O → [ R - NH- C- OH] → R - NH -C -NH - R + CO2


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(5) Amides react to form acyl urea

O O || ||

R - N = C = O + R� C O NH2 → R -NH - C - NH - C -R�

Acyl urea (11)

(6) Carboxylic acids: substituted amides are formed with liberation of carbondioxide

O O O || || ||

R - N = C =O + R�COOH -→ [ R - NH - C -O -C - R�] → R-NH-C-R� + CO2

Substituted amide


(7) Self-addition reactions of isocyanate: highly reactive aryl isocyanatesdimerize in the presence of catalysts to form uretidinediones. The for-mation of uretidinedione results in loss of reactivity of the isocyanate instorage. The uretidinedione formation is a means to block isocyanates andto make the isocyanate group available at elevated temperatures. Catalyzedby strong bases, isocyanates also undergo trimerization to form isocya-nurate ring structure, which is very stable toward heat and most chemi-cals. In the case of polyisocyanate, highly branched polyisocyanurates areformed.

R - N =C =O + O = C = N -R →








(13) Important Polyisocyanates and Their Synthesis

The most important route for the synthesis of isocyanates is the phosgenationof primary amines.

R-NH2 + COCl2 → R-N=C=O + 2 HCl (14)

Toluene diisocyanate (TDI) is one of the most important diisocyanates. Thesynthetic steps are dinitration of toluene, reduction to the corresponding di-amines, followed by phosgenation to yield TDI. It is used as a mixture of 2,4and 2,6 isomers in the ratio of 80:20. The two isomers differ considerably inreactivity, so the actual ratio of the two components is quite important.

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Figure 1.6 Isomers of MDI.

Another important aromatic diisocyanate is diphenyl methane diisocyanate(MDI). Condensation of aniline with formaldehyde leads to a mixture of4,4′-, 2,2′- and 2,4′-diamino diphenyl methanes as well as polyamines. Theseon phosgenation form the corresponding isocyanates (Figure 1.6).

Other common aromatic isocyanates used are naphthalene 1,5-diisocyanate,xylelene diisocyanate XDI, p-phenylene diisocyanate PPDI, and 3,3′-tolidinediisocyanate TODI.

Aromatic isocyanates yield polyurethanes that turn yellow with exposure tolight. Various aliphatic and cycloaliphatic diisocyanates are used in the industryto produce polyurethanes, which do not turn yellow upon light exposure. Theseare extensively used for coatings. The most important among the aliphaticsis hexamethylene diisocyanate (HMDI) obtained by the reduction of adiponi-trile and phosgenation of hexamethylene diamine. Among the cycloaliphatics,isophorone diisocyanate (IPDI) is the most common. Others are cyclohexyldiisocyanate (CHDI), 4,4′-dicyclohexyl methane diisocyanate (H12MDI), and2,2,4-trimethyl-1,6-hexamethylene diisocyanate (TMDI).

OCN-(CH2) 6 -NCO



NCO Blocked Isocyanates

In blocked isocyanates, the isocyanate groups are reacted with compoundsto form a thermally weak bond. On heating, the bond dissociates to regenerate

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the isocyanate group. The most common example is the reaction product ofphenols to yield aryl urethanes that dissociate at ∼150◦C.

R-N=C=O + ArOH → R-NH-COOAr (15)

Similar adducts are formed by the reaction of isocyanates with dipheny-lamine, succinimide, acetoacetic ester, oximes, triazoles, caprolactams, etc.Dimers of isocyanates can also be considered as blocked isocyanate. The ringopening can be thermal or catalytic, without the liberation of any volatile block-ing agent.













The blocked isocyanates are specifically used in one component systemsfor coatings adhesives. They are also used for preparing aqueous polyure-thane dispersions. The blocked isocyanate group does not react with water.The isocyanate group is regenerated for reaction after drying and heating of thedispersion. Polyols

Besides the polyisocyanates, the other important building blocks of polyure-thanes are polyfunctional alcohols. Polyurethanes made from short-chain diolsyield linear crystalline fiber-forming polymers, lower melting than the corre-sponding polyamides, that are of little commercial interest. However, reactionof isocyanates with polymeric glycols lead to the formation of polyurethanesof diverse physical and mechanical properties, suitable for a variety of appli-cations. The molecular weight of the polyols ranges from 200 to 10,000. Twotypes of polyols are used in the synthesis of polyurethane: polyester polyolsand polyether polyols containing at least two hydroxyl groups in a polyester orpolyether chain. Polyester Polyols

These are saturated dicarboxylic esters, the reaction product of dibasic acidand a diol. The commonly used acids are adipic, phthalic, sebacic, and dimer

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acids (dimerized lineoleic acid). The diols are ethylene glycol, diethyleneglycol, triethylene glycol, 1,2-propylene glycol, etc. For higher functionality,glycerol, trimethylol propane, penta erythritol, sorbitol, etc, are used wherechain branching and higher cross-linking are required.

For preparation of the polyester, conventional methods of polyesterificationare used. The molecular weight can be controlled by the molar ratio of the reac-tants and the reaction conditions, however, it is essential that the terminal groupsbe hydroxyl for reaction with the isocyanate. For this purpose, esterification iscarried out with stoichiometric excess of the diol.


Polyester polyol

Caprolactone polyester formed by polyaddition with a diol and ε-caprolactonein the presence of an initiator is also of commercial interest. The advantage ofthis reaction is that no water is formed.

O HO-R-OH ||

n → HO-R-O-[C-(CH2) 5-O-] n - H

Caprolactone polyester




(17) Polyether Polyols

Polyether polyols are also known as polyalkylene glycols or polyalkyleneoxides. The common polyether polyols are polypropylene glycols and poly-tetramethylene glycol (Figure 1.7).

Polypropylene glycol, the most commercially important polyol, is preparedby a base-catalyzed ring opening polyaddition reaction. However, polytetra-methylene glycol is obtained by acid-catalyzed ring opening polyaddition.

Block copolymers of polyethylene and polypropylene glycols are commonlyused in the industry. These are obtained by reacting ethylene oxide with polypro-pylene glycol or propylene oxide with polyethylene oxide in the presence of abase catalyst. This offers a means of adjusting the ratio of primary and secondaryhydroxyl groups. The general formula of the block copolymer is given by thefollowing:


HO-( CH2-CH2-O-)a- (CH2-CH-O-)b-(CH2-CH2-O-)c-H

Copolymer of ethylene and propylene oxide

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Figure 1.7 Common polyether polyols. Cross-linkers and Chain Extenders

These are low molecular weight polyfunctional alcohols and amines thatact as chain extenders or cross-linkers by reaction with the -NCO group. Thealcohols form urethane, and the amines form urea linkages. The difunctionalcompounds are essentially chain extenders, while the compounds with function-ality greater than two are cross-linkers. The end properties of the polyurethanesare considerably influenced by these compounds, as they alter the hard to softsegments proportion of the polymer. Some important diol chain extenders areethylene glycol, 1,4-butanediol, etc. Trifunctional alcohols like glycerol andtrimethylol propane act as cross-linkers. Among the amines, derivatives of di-amino phenyl methane and m-phenylene diamines are of commercial interest aschain extenders. The most common in this category is 3,3′dichloro 4,4′diaminodiphenyl methane (MOCA). Catalysts

The rate of the reactions of isocyanates can be greatly enhanced by usingappropriate catalysts. The most important catalysts are the tertiary aminesand organo tin compounds. The rate increase of urethane link formation by-NCO/OH reaction depends on the basicity and the structure of the amines.Some important amines are triethyl amine, triethylene diamine and peralkylatedaliphatic polyamines. Prominent among the organo tin compounds are dibutyltin dilaurate and tin dioctoate. They are readily soluble in the reaction mixtureand have low volatility and little odor. Basicity favors formation of branchingand cross-linking. Commercially, a mixture of amine and tin catalysts are usedfor synergistic effect.

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There are two methods of preparation of polyurethanes.

(1) One-shot process: in this process, the entire polymer formation takes placein one step by simultaneously mixing polyol, diisocyanate, chain extender,and catalyst. The reaction is very exothermic and requires similar reactivi-ties of different hydroxy compounds with the isocyanate.

(2) Prepolymer process: this is a two-stage process. In the first stage, diiso-cyanate and polyol are reacted together to form an intermediate polymer ofa molecular weight of about 20,000, which is called a prepolymer. Depend-ing on the stoichiometry of the diisocyanate and polyol, the prepolymer canbe NCO terminated or OH terminated. The NCO-terminated prepolymersare of great technical importance, as the NCO groups are available for reac-tion with compounds containing active hydrogen atoms. The prepolymer isthen reacted with a chain extender to form the final high molecular weightpolymer, either by a polyfunctional alcohol or amine (Figure 1.8).


Polyurethanes prepared from short-chain diols and diisocyanate have a largeconcentration of urethane linkage which results in a high degree of hydrogenbonding between the -NH and C =O groups of the chains. Consequently, thesepolymers are hard and have a low degree of solubility. On the other hand, thereaction product of long-chain polyols and diisocyanates results in polymerswith a low concentration of urethane groups. The intermolecular forces are,therefore, mainly weak van der Waals forces, and the polyurethane is low inhardness and strength. Most polyurethanes are prepared from at least threebasic starting materials, viz., (1) long-chain polyol, (2) diisocyanate, and (3) achain extender. These linear chains of polyurethane elastomer show segmentedstructure (block copolymer), comprised of an alternate soft segment of thepolyol with weak interchain interaction, present in coiled form, and a hardsegment formed by reaction of diol/diamine and the diisocyanate (Figure 1.9).The hard segments have strong interchain H-bonding and dipolar interactionsdue to the presence of a large number of polar groups—urethanes and ureas.

The hard and soft segments are partly incompatible because of their differ-ence in polarity; as such, they show two-phase morphology. The hard segments

Figure 1.8 Prepolymer process.

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Figure 1.9 Segmented polyurethane.

form discrete domains in a matrix of soft segments. The aggregated hard seg-ments tie the polymeric chains at localized points, acting as cross-links andas reinforcing filler matrix in a soft segment matrix (Figure 1.10). The two-phase morphology is applicable to linear elastomers as well as to most of thecross-linked polyurethanes.


TPUs are high molecular weight polymers obtained by the reaction of polyol,diisocyanate, and chain extender. These are fully reacted linear chains, seg-mented in structure with hetero-phase morphology as described in the pre-vious section. If however, stoichiometric excess of isocyanate is taken, i.e.,NCO/OH > 1.0, the free NCO groups react with urethane groups formingallophanate branched or cross-linked structures.

The cross-links formed due to hard segment domains impart elasticity to thepolymer, however, these are reversible to heat and solvation, permitting thermo-forming and solution application of TPU. They can, therefore, be termed vir-tual cross-links. The soft matrix has Tg lower than room temperature, and isamorphous in nature, but the hard segments are paracrystalline or crystalline.

The property of the elastomer is dependent on the type of polyol, molecularweight, and the ratio of hard to soft segments. The molecular weight of thepolymer for optimum physical properties is between Mw 100,000–200,000.Polyethers generally give elastomers having a lower level of physical propertiesthan the polyester polyols. The elastomer can be compounded in a plastic orrubber mill. Stabilizers, processing aids, and extenders are the additives used.They do not require any curing.

Figure 1.10 Two-phase morphology of polyurethane.

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The conventional solution-based coatings are of two types, viz., one-component systems and two-component systems. One-Component System

These are two types of one-component systems: reactive and completelyreacted systems.

a. Reactive one-component systems: these systems are low molecular weightprepolymers with terminal isocyanate groups. They are dissolved in solventsof low polarity. After coating, they are moisture cured. The water acts as achain extender and cross-linking agent with the formation of urea and biuretlinkages. The generation of carbon dioxide is sufficiently slow, so that slowdiffusion of the gas from the film occurs without bubble formation. The rateof cure is dependent on the temperature of the cure and the humidity ofthe ambient. Use of blocked isocyanate prepolymers allows formulations ofone-component systems that are stable at room temperature.

b. Completely reacted one-component system: this consists of totally reactedhigh molecular weight thermoplastic polyurethane elastomers. The PU isdissolved in a highly polar solvent like dimethyl formamide. These coatingsdry physically. Two-Component System

In this type of coating system, isocyanate-terminated prepolymers or poly-functional isocyanates are reacted with polyhydroxy compounds that may al-ready be urethane modified. The polyisocyanate component, usually in the formof a solution, is mixed with the polyhydroxy component prior to coating. Curingof these coatings occurs due to the formation of urethane linkages. In addition,reaction with moisture also takes place. The properties of the resulting coatingsdepend on various factors, viz.,

(1) The polyol type and molecular weight

(2) The temperature of the reaction

(3) The concentration of polar groups, i.e., urethane and urea

(4) The cross-linking density

In the U.S., for PU-coated fabrics, TPU systems are preferred. By varyingthe polyol and NCO/OH ratio in the TPU manufacturing process, the samewide range of flexibility can be obtained as mentioned for the two-componentsystems. Proper choice of TPU adhesive is critical. It is common practice to add

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a polymeric isocyanate to the TPU adhesive layer to aid in adhesion to the basefabric. There is almost no transfer coating done in the U.S. today for wearingapparel. It is all done overseas. In the U.S., fabrics for tenting, recreationalclothing, etc., are direct coated with TPU solvent system.

Solvent-free coating by TPU elastomers: these can be coated on the fabricby hot melt process of the solid polymer. The common method employed isthe Zimmer coating method. The PU can also be extrusion fed to a Bema or acalender. Additives

The additives used for urethane coatings are generally silica fillers to reducegloss, UV absorbers, antioxidants, and flow improvers. The solvents used forcoating should be free of moisture and reactive hydrogen to prevent reactionwith free isocyanate in two-component systems. They are generally polar innature, and care should be taken for their selection to ensure storage stabilityand blister-free film. Similarly, the pigment used should also be moisture free.


In recent years, there has been a trend to use PU latices for coating for thefollowing reasons.

� low emission of organic volatiles to meet emission control regulations� lower toxicity and fire hazard� economy of the solvent� viscosity of latex independent of molecular weight

Chemically PU latices are polyurethane-urea elastomers dispersed in water.The starting materials are polyether/polyester polyol, diisocyanates, and poly-functional amines-chain extenders. Isocyanates should have low reactivity towater, as the carbon dioxide produced leads to foaming. For chain extensionpurposes aliphatic or cycloaliphatic amines are preferred, because they reactfaster with the isocyanate group. Emulsifiers

Hydrophobic polyurethane can be dispersed in water with a protective colloid.Alternately, hydrophilic groups can be incorporated in the polymer chain byinternal emulsifiers that have the following advantages:

a. Dispersion does not require strong shear force

b. Leads to better dispersion stability

c. Finer particle size

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These emulsifiers are of two types: ionic and nonionic. Ionic internal emul-sifiers are anionic or cationic groups built in the polymer chain. Common ionicemulsifiers are as follows:

(1) Sulfonated diamine: H2 N-CH2-CH2-- NH-CH2-CH2-SO3- Na+

(2) Sulfonated diols: HO-CH2-CH2-CH-CH2-OHSO3

- Na+


(3) Dihydroxy carboxylic acids: HO-CH 2-C-CH2 OH


(4) A tertiary amine: HO-CH2-CH2-N-CH2-CH2-OH


Polyurethane ionomers with built in ionic/hydrophilic groups are obtainedby reacting NCO terminated prepolymer with ionic internal emulsifiers. Non-ionic internal emulsifiers are polyether chains of polyethylene oxide, whichare incorporated in the PU chain. These segments, being hydrophilic in nature,act as emulsifiers of the elastomer. The disadvantage of such emulsificationis the water sensitivity of the dried film. Anionic dispersions are more widelyused.

Typically, polymer particle size ranges from 0.01 to 0.1 micron. In the ioni-cally stabilized dispersions, the ionic centers are located at the surface, and thehydrophobic chain segments are at the interior. They are sensitive to electrolytes.Nonionically stabilized dispersons are sensitive to heat, as polyethylene glycolpolyol lose their hydrophilicity at higher temperatures. Preparation of Dispersions

There are different methods for preparing dispersions [12,18], e.g., acetoneprocess melt dispersion process, etc. Acetone Process

A solution of high molecular weight polyurethane-urea ionomer is builtup (after reaction and chain extension) in a hydrophilic solvent like acetone,dioxane, or tetrahydrofuran. The solution is then mixed into water. On removalof the solvent by distillation, an aqueous dispersion of the polymer is obtained

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Figure 1.11 Acetone process of PU dispersion.

by phase inversion of the emulsion originally formed with the water-organicsolvent. A typical reaction sequence is shown above (Figure 1.11). Melt Dispersion Process

A -NCO terminated prepolymer is reacted with ammonia or urea to formurea or biuret end groups. The reaction with urea is carried out at a hightemperature ∼130◦C. The hot melt is poured into water at an elevated tem-perature to form a spontaneous dispersion. The end capping of -NCO groupsrenders them nonreactive to water. Chain extension is carried out by reactionof the oligomer with formaldehyde through the formation of methylol groupsat the biuret functionality at a lower pH. The reaction sequence is shown inFigure 1.12.

Figure 1.12 Melt dispersion process.

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On drying of the dispersion on a substrate, the discrete polymer particlesshould fuse to form a continuous organic phase with entanglement of polymerchains. Poor fusion leads to poor gloss and poor physical properties of the film.If cross-links are present, the film-forming property decreases. An improvementcan be made by adding high boiling, water miscible solvent in the latex. Onevaporation of water, a solution of PU in the solvent is left behind. The solventon evaporation gives a continuous film. A commonly used solvent is N-methylpyrrolidone.

The main drawbacks of PU dispersion compared to two-component solutionare the poor solvency and water resistance. Improvement in these propertiescan be obtained two ways:

(1) Grafting hydrophobic chains, usually acrylics, on the PU backbone

(2) Cross-linking of the polyurethane chains particularly those containing car-boxylate ion, using polyfunctional aziridines

For coating purposes, certain additives are added in dispersion. These in-clude thickening agents (e.g., polyacrylate resins), extenders, pigments, flameretardants, and external cross-linking, e.g., aziridines and melamine resins.

Great improvements have been made in water-based polyurethane chemistry;however, the wide flexibility of properties that can be obtained by solvent-basedsystems has not been demonstrated with the water-based coatings. To date,where strength, flexibility, toughness, etc., are the required physical propertiesof the end product, solvent-based systems are the coatings of choice.


Polyurethane coating on textiles gives a wide range of properties to the fabricto meet diverse end uses like apparel, artificial leather, fuel and water storagetanks, inflatable rafts, containment liners, etc. This is because of a wide selectionof different raw materials for their synthesis. For a breathable microporouscoating, PU is the polymer of choice. PU-coated fabric offers advantages, whichare given below, over other polymeric coatings [20,22,23].

� dry cleanability, as no plasticizers are used� low temperature flexibility� overall toughness—very high tensile, tear strength, and abrasion

resistance requiring much less coating weight� softer handle� can be coated to give leather-like property and appearance

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As has already been discussed, polyurethanes show two-phase morphologyof soft and hard segments. The types and content of these segments profoundlyinfluence the properties of the polyurethane. The soft segments obtained fromthe polyol determine the elastic and low temperature properties. Increasingthe molecular weight of the polymer by increasing the soft segments resultsin lowering of the mechanical property, making the polymer softer and moreextensible. Similarly, the presence of pendant side groups in the polyol alsoresults in lowering of the physical properties. Increase in hard segment in thepolymer increases its hardness, tensile modulus, and tear resistance. Diaminechain extenders form urea linkages, that enter into stronger hydrogen bondsand yield stiffer hard segments, thus, the PU formed has higher hardness andmodulus than that obtained by diol extenders.

Polyester polyurethanes generally show higher modulus, tensile strength,hardness, and thermal oxidative stability than the polyether urethanes. Thisis because of the higher cohesive energy of the polyester chains. They alsoshow better resistance to hydrocarbons, oils, and greases, but they show poorerhydrolytic stability due to the ester linkages. The hydrolytic stability of thepolyester polyurethane can be increased by using sterically hindered glycolslike neopentyl glycol and long-chain or aromatic diacid-like terephthalic acid.

Hydrolytic stability increases with hydrophobicity of the chain, thus, poly-ether polyurethanes have better hydrolytic stability. The thermal oxidative sta-bility of polyether polyurethanes can be improved by adding antioxidants.Polyether urethanes also show better resistance to mildew attack.

The structure of isocyanates also influences the properties of the polyurethane.Symmetrical aromatic diisocyanates like naphthalene diisocyanate, diphenylmethane diisocyanate, and p-phenylene diisocyanate give a harder polymerwith a higher modulus and tear strength compared to those obtained using lesssymmetrical ones such as 2,4- and 2,6-TDI. Polyurethanes of aliphatic andcycloaliphatic diisocyanates are less reactive and yield PU that have greaterresistance to UV degradation, and thermal decomposition. They do not yellowon weathering but have a lower resistance to oxidation. The yellowing ofpolyurethanes containing aromatic diisocyanates is due to UV-induced oxi-dation that results in the formation of quinone imide [27] (Figure 1.13).

Figure 1.13 UV-induced oxidation of MDI.

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They are commonly known as acrylics. The monomers are esters of acrylicand methacrylic acid.

Acrylic ester







This is the general formula of acrylates (R = H for acrylates, R = CH3

for methacrylates). Some common esters are methyl, ethyl, n-butyl, isobutyl,2-ethyl hexyl, and octyl. The esters can contain functional groups such as hy-droxyl, amino, and amido. The monomers can be multifunctional as well, suchas trimethylol propane triacrylate or butylene glycol diacrylate. The nature ofthe R and R′ groups determines the properties of monomers and their polymers.Polymers of this class are noted for their outstanding clarity and stability of theirproperties upon aging under severe service conditions.

Polymerization of the monomers occurs by free radical polymerization usingfree radical initiators, such as azo compounds or peroxides. Acrylic polymerstend to be soft and tacky, while the methacrylate polymers are hard and brittle.A proper adjustment of the amount of each type of monomer yields polymersof desirable hardness or flexibility. A vast majority of commercially availableacrylic polymers are copolymers of acrylic and methacrylic esters. The poly-merization can occur by bulk, solution, emulsion, and suspension methods.The suspension-grade polymer is used for molding powders. The emulsion andsolution grades are used for coatings and adhesives.

Acrylate emulsions are extensively used as thickeners and for coatings.Acrylics have exceptional resistance to UV light, heat, ozone, chemicals, water,stiffening on aging, and dry-cleaning solvents. As such, acrylics are used asbackcoating materials in automotive upholstery fabric and carpets, windowdrapes, and pile fabrics used for outerwear.



The adhesion of the polymer to the textile substrate is an important aspectof coating technology, especially when the articles are put to dynamic use. Themain mechanisms of adhesion are as follows [28]:

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(1) Mechanical interlocking: this mechanism operates when the adhesive in-terlocks around the irregularities or pores of the substrate, forming a me-chanical anchor. A rough surface has a higher bonding area.

(2) Adsorption: the attractive forces may be physical, i.e., physical adsorptionby van der Waals forces, H-bonding, or chemical bonding (chemisorption).

(3) Diffusion: the adhesive macromolecules diffuse into the substrate, i.e., in-terpenetration occurs at the molecular level. It requires that the macro-molecules of the adhesive and the adherend have sufficient chain mobilityand are mutually compatible.

The irregularities on the textile substrate for mechanical interlocking of theelastomer are fiber ends, twists, crimps of the yarn, and interstices of the weavepattern. Cotton fabric and yarns made from staple fiber have a much highersurface area, and the fiber ends become embedded in the elastomeric matrix.In contrast, the synthetic fibers normally produced in continuous filament aresmooth and, hence, have relatively poor adhesion.

All textiles of practical interest have surfaces that contain oriented dipolesthat induce dipoles in the elastomer. As such, the contribution of dipole-inducedinteractions is quite prominent. Hydrogen bonding also provides significantcontribution to many coatings. Chemical bonding due to formation of cross-links is also important, particularly when keying agents or special adhesivetreaments are given to the fabric.

Textile substrates made from cotton or a high proportion of cotton do notgenerally require an adhesive treatment because the mechanical interlockingof the staple fiber ends into the elastomeric matrix imparts adequate adhesion.Rayon, nylon, and polyester used mainly in continuous filament forms require anadhesive pretreatment. The type of bonding systems in use are the resorcinol-formaldehyde-latex (RFL) dip systems, dry bonding systems and isocyanatebonding.


This system is used for adhesive treatment of rayon and nylon. The laticesused are generally of natural rubber, SBR, or vinyl pyridine copolymer (VP). Atypical method of preparation consists of mixing resorcinol and formaldehydein the molar ratio of 1:1.5 to 1:2.5 (in an alkaline medium resole process) ina rubber processer. The mixture is stored for about 6 hrs at room temperature.To this is then added appropriate latex or latex mixture. This mix is maturedfor 12–24 hrs prior to use. Many factors affect the performance of the RFL dipsuch as (1) the resorcinol-to-formaldehyde ratio (2) the pH and conditions ofreactions, (3) the type of latex used, (4) the ratio of latex to resin, and (5) totalsolids, etc. A SBR-VP latex mixture is commonly used for rayon, and nylon.For standard tenacity rayon, the ratio of SBR to VP latex is about 80:20, but for

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higher tenacity yarns, a higher ratio (50%–80%) of VP latex is used. The solidcontent of the dip is adjusted to the type of fiber used, e.g., about 10–15 wt.%for rayon and about 20 wt.% for nylon. For nylon 6 and 66, better adhesion isachieved at 75% VP latex or higher.

The process of application consists of impregnating the fabric by passingthrough a bath of the dip. Excess dip is removed by squeezing through rollers.The water is then removed by drying at around 100–120◦C. The treated fabricis then baked by heating at temperatures ranging from 140–160◦C for a shortperiod of time (1–2 mins). Total solid pickup is controlled mainly by solidcontent of the dip. The add-on required depends on the fiber. For rayon, theadd-on ranges from 5–8 wt.%, for heavier nylon fabrics, an add-on of up to15% may be required. In the case of nylon, curing is combined with heat settingat 170–200◦C. The baking enhances adhesion due to increased condensation ofthe resin and creation of more reactive sites [31].

The RFL dip discussed above is adequate for olefin rubbers. For polymerslike CR, NBR, PVC, IIR (butyl), and EPDM, the RFL dip has to be modifiedfor better adhesion. With CR and NBR, replacement of latex of the RFL dip by50–100% of the latex of the corresponding polymer gives satisfactory results.Latices of IIR and EPDM do not exist, although their emulsions are available thatcan be used to substitute the latex of RFL dip. However, the performance withemulsions is not satisfactory. For PVC, the latex has to be carefully selected, asall PVC latices do not form coherent film, and it may be desirable to incorporatean emulsion of the plasticizer in the dip for proper film formation.

Polyester fabric requires a two-stage dip. In stage one, the fabric is dipped intoan adhesive consisting of water-miscible epoxy (derived from epichlorohydrinand glycerol) and a blocked isocyanate dispersion, to give a pickup of about0.5%. The blocked isocyanate is activated at about 230◦C. In stage two, thefabric is dipped in a standard RFL system. A two-stage dip is also used foraramid fabrics.

Estimation of add-on of RFL dip is critical for determining the adhesion level.The conventional ASTM method based on acid extraction of the coated materialis time consuming. Faster results have been reported using near infrared (NIR)reflectance measurement [32].

1.5.3 DRY BONDING SYSTEM [3,30,33]

In this system, the adhesion promoting additives are added to the elastomercompound itself. Adequate bonding is achieved by coating the compound onuntreated textile substrate. The important ingredients of the system are resor-cinol, a formaldehyde donor-hexamethylene tetramine (HMT), and hydratedsilica of fine particle size. The resorcinol formaldehyde resin formation takesplace during the vulcanization process, which then migrates to the rubber-textile inerface, resulting in an efficient bond between the two surfaces. Therole of silica is not fully clear. It probably retards the vulcanization process,

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allowing time for the formaldehyde donor to react with resorcinol and the resinto migrate to the interface for bond formation. The process is universally appli-cable to all types of rubber in combination with all types of textile materials.With polyester, however, hexamethoxymethyl melamine (HMMM) is used asa methylene donor instead of HMT, as the amine residue of HMT degradesthe polyester by ammonolysis of the ester linkage. Because this system actsby migration of the resin components, a minimum thickness of the adhesivecompound is required at the interface to prevent back migration of the ad-hesive components to the bulk. The normal amounts of resorcinol and HMTadded are around 2.5 phr and 1.5 phr, respectively, however, the concentra-tion is dependent on the formulation of the compound and the type of fabric.For dry bonding, the composition of the rubber compound should be carefullybalanced. Nonsulfur curing systems lead to poorer adhesion. Use of ultraac-celerators is unfavorable because they often do not give sufficient time for therelease of the required amount of formaldehyde for resin formation. On the otherhand, too much delay in curing also leads to poor adhesion. Accelerators likeN-cyclohexyl-2-benzothiazyl sulfonamide (CBS), used alone or in combina-tion with basic secondary accelerators like N,N-diphenyl guanidine (DPG),give good adhesion.


The mechanism of action of the RFL system has been investigated. The RFLfilm forms a bond with the coated elastomer as well as the textile substrate.The dip film to elastomer bond occurs due to cross-linking of the rubber (latex)part of the film by migration of sulfur and curatives from the main elastomer. Aminor contribution is due to the reaction of the resin part of the dip film with theactive hydrogen atom of the main elastomer, forming a chroman-like structure,as given below [30] [Equation (18)]. Such a mechanism and formation of aninterpenetrating network explains the adhesion of the dry bonding system.

+ ~~~~~C CH



R" O









Chroman-like structure


The adhesion of the textile substrate with the dip is believed to be due toits resin component. Various mechanisms are operative; they are mechanical

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interlocking, diffusion, and chemical bond formation. The methylol group ofresins reacts with the hydroxyl groups of the cellulosics and amido group ofnylon forming covalent bonds. This mechanism is applicable to the dry bondingsystem as well. In the case of polyester, the blocked isocyanate of the two-stagesystem forms a polyurethane with a solubility parameter similar to that ofpolyester, thus favoring adhesive bond by diffusion. The new surface is reactivetoward the second-stage RFL dip [30].


Polyisocyanates are used for binding elastomer and fabric. The isocyanategroups are highly reactive, and they bond elastomer and textile by reactingwith their reactive groups. The common isocyanates used are 4,4,4-triphenylmethane triisocyanate (TTI), diphenyl methane diisocyanate (MDI), diani-sidine diisocyanate (DADI), and polymethylene polyphenyl isocyanate (PAPI).There are two general procedures, viz., the solution process and the doughprocess [13]. In the solution process, a dilute solution of the polyisocyanate(∼2% concentration) in toluene or methylene dichloride is applied on the fab-ric by spraying or dipping. After evaporation of the solvent, an elastomericcoating is applied in the usual manner. In the dough process, compoundedrubber stock is dissolved in a suitable solvent like toluene, chlorobenzene, orgasoline, and is then mixed in a sigma mixer. To this solution (cement) is addedisocyanate solution with agitation. This solution is then applied on the fabricusing a conventional method to an add-on of about 10–15% and then dried.This coating acts as a primer for adhesion of the elastomeric coat to the fabric.The treated fabric in this case is better protected from moisture than in thesolvent process. The composite-coated fabric can be cured in the usual manner.Isocyanate bonding agents give coated fabrics better softness and adhesion thanthat provided by the RFL system. The use of blocked isocyanate along with therubber dough increases the pot life of the adhesive dip.

The adhesive treatments discussed in this section are primarily for elastomericcoatings on fabric. The formulation systems for other polymers and the factorsresponsible for proper adhesion have been discussed in Chapter 4.



Conventionally, curing of a polymer composition is done by thermal energyfrom sources such as electrical heaters, high pressure steam, hot air from elec-tric heaters and infrared heaters. Curing by heat generated by a microwave isalso being used for continuous vulcanization of rubber compounds. Radiation

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curing, i.e., curing by ultraviolet and electron beam radiation, does not involveheating, instead ions or free radicals are generated and the macroradicals sogenerated couple together to produce a three-dimensional network. In bothUV/E beam, the formulation used for coating consists of an oligomer, a reac-tive monomer, and in the case of UV curing, a photoinitiator. Radiation-curedcoatings have several advantages [34–36].

� fast curing speeds� high solid content—usually 100% solids� compact curing lines and decreased floor space� low capital investment� ability to cure heat-sensitive substrates� wide variety of formulations� reduced pollution� lower energy consumption


The chemistry discussed here is for ultraviolet radiation technology. In ageneral sense, the same formulations are used for both UV and E beam, exceptthat in the case of UV curing, a photoinitiator is required to initiate free radicalformation. The formulation of UV curing consists of the following [35–37]:

(1) A monomer and/or an oligomer bearing multifunctional unsaturated groups

(2) A photoinitiator that must effectively absorb incident UV light and produceinitiating species with high efficiency Photoinitiators

The different photoinitiators can be classified into three major categories:

(1) Free radical formation by homolytic cleavage: Benzoin alkyl ethers, benzilketals, and acetophenone derivatives belong to this class. Here, the pho-toinitiator undergoes fragmentation when exposed to UV light. The ben-zoyl radical is the major initiating species in the cleavage of benzoin alkylether. Cleavage of benzoin methyl ether is shown in Equation (19).

Benzoin alkyl ether Benzoyl radical Methoxy benzyl radical


_ _ __ _C


C CO +_







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(2) Radical generation by electron transfer: This mechanism involves pho-tolytic excitation of the photoinitiator followed by electron transfer to ahydrogen atom donor, a tertiary amine [Equation (20)].

decays to initiating free inert species radical






(20)Typical photoinitiators in this category are 2,2-diethoxyacetophenone,2,2-dimethoxy-2-phenyl acetophenone, etc.

(3) Cationic type: Aryl diazonium salts PhN2 +X− undergo fast fragmentation

under UV radiation with the formation of free Lewis acids, which are knownfor cationic cure of epoxides [Equation (21)]

PhN2 +BF4

− hν→ PhF + N2 + BF3 (21) Polymer Systems

One of the early UV curable systems was based on unsaturated polyesterand styrene. The unsaturation located in the polymer chain undergoes directaddition copolymerization with the vinyl group of the monomer leading to across-linked network (Figure 1.14).

Multifunctional acrylates are the most widely used systems. The oligomeris usually a urethane or epoxy chain end capped on both sides by acrylategroups. The molecular weight ranges from 500–3000. Reactive diluents areadded to lower the viscosity of the oligomers and to increase the cure rate. Thereactive diluents are generally mono- or multifunctional acrylate compoundswith a molecular weight less than 500. A reaction sequence is given below(Figure 1.15).

Cationic polymerization of epoxides is another method used. As discussedearlier, Lewis acids promote ring opening polymerization of epoxides, lactones,or acetals.

The free-radical-induced polymerization is inhibited by oxygen. Severalmethods have been developed to reduce the undesirable effects of oxygen.

Figure 1.14 Unsaturated polyester system.

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Figure 1.15 Cross linked acrylate system.

Some of these are as follows:

� increase of UV lamp intensity� optimization of the photoinitiator system� curing in an inert atmosphere� addition of oxygen scavengers


(1) Ultraviolet light: different types of UV curing technologies are known.They are medium pressure mercury vapor lamp, electroless vapor lamps,and pulsed xenon lamps.

(2) Electron beam: the source of electrons is a tungsten filament that is insidea vacuum tube. The electrons are accelerated by the application of highvoltage, 150,000 to 300,000 V. The accelerated electrons pass through ametallic foil window and are directed on the polymer meant for curing.X-rays are generated along with the electrons; therefore, it is necessary toshield the entire housing of the EB equipment. The energy received by thisformulation is known as a dose and is termed megarad (1 Mrad = 10 joules).


Radiation cure has been in use in graphic arts, inks, printing, laminating,packaging, and in the electronic industry [36–38]. Motivated by the signifi-cant advantages of radiation-cured coatings over the conventional solvent-basedthermal cure systems, Walsh and coworkers carried out an extensive study ofradiation-cured coatings for textiles for different end uses. The work has beenreported in a series of publications [39–43]. One application investigated wasthe backcoating of upholstery fabric [39], usually done by a thin latex coating,to stabilize the fabric against distortion and yarn raveling. A comparative studywas done on nylon-viscose upholstery fabric. The latex coating was carried outby spraying and thermal curing, while the UV coating formulation was transfercoated on the fabric and cured by UV radiation. The flexural rigidity and yarn

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raveling tests showed that UV-cured samples were comparable to the latex-coated samples. A cost analysis showed that UV curing was only economicalat low weight add-on (∼2%). Studies were also conducted to achieve thickercoatings for synthetic leather applications, by radiation cure process, in lieu ofthe conventional solution-based polyurethane coating [40]. A suitable EB cureformulation consisting of acrylated urethane oligomer and acrylate monomerwas coated on woven and knitted textile substrates by transfer coating and curedby electron beam (5 Mrads). The transfer coating was done by different ways.

None of the coated specimens, however, passed the flex fatigue test requiredfor apparel fabric. This is largely due to the high degree of cross-linking in thecured film. A novel application studied by the authors was the simultaneouscoating on both sides of nonwoven fabric by EB cure, in order to improvethe durability and aesthetics of the coated fabric [41]. Simultaneous coating onboth sides of the fabric is not possible in conventional solution-based processes.Essentially, the coating formulation was cast on two release papers and transfercoated on nonwoven substrate on both sides. This sandwich construction wasthen cured by electron beam.

Walsh et al., [42,43] continued their studies on the mechanical properties ofEB-cured films using polyester acrylate urethane oligomers of different molec-ular weights and different reactive monomers with a view to developing suitablecoatings for textiles. It was found, that higher molecular weight of the oligomerlowers the modulus of the film, the Tg , and the breaking strength. Addition of achain transfer agent improved toughness as well as extensibility. Studies havealso been carried out at TNO laboratory Holland [37,44]. They have carried outstudies on UV curable coatings and the development of UV curable binders forpigment printing. Pigments have to be carefully selected so that photoinitiatorsand the pigment have different absorption characteristics, otherwise, insufficientcuring occurs and bonding is poor.

The use of radiation cure for textiles has not yet become popular, because fewsystems can meet the requirements of textile coating, i.e., flexibility, strengthof the film, bonding, and chemical resistance [37].


1. Rubber Technology and Manufacture, C. M. Blow, Ed., Butterworths, 1971.2. Science and Technology of Rubber, F. R. Eirich, Academic Press, NewYork, 1978.3. Rubber Technology Handbook, W. Hoffmann, Hanser Publishers, Munich, 1989.4. Rubber Chemistry, J. A. Brydson, Applied Science, London, 1978.5. Advances in silicone rubber technology, K. E. Polmanteer, in Handbook of Elas-

tomers, A. K. Bhowmik and H. L. Stephens, Eds., Marcel Dekker, 1988, pp. 551–615.

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6. J. Schwark and J. Muller, Journal of Coated Fabrics, vol. 26, July, 1996, pp. 65–77.7. Encyclopedia of PVC, vol. 1–3, L. I. Nass, Ed., Marcel Dekker Inc, New York,

1977.8. PVC Plastics, W. V. Titow, Elsevier Applied Science, London and New York, 1990.9. Polyvinyl Chloride, H. A. Sarvetnick, Van Nostrand Reinhold, New York, 1969.

10. PVC Technology, A. S. Athalye and P. Trivedi, Multi Tech Publishing Co., Bombay,1994.

11. Manufacture and Processing of PVC, R. H. Burges, Ed., Applied Science Publishers,U.K, 1982.

12. Polyurethane Handbook, G. Oertel, Hanser Publishers, Munich, 1983.13. Polyurethane Elastomers, C. Hepburn, Applied Science Publishers, London and

New York, 1982.14. Polyurethane Chemistry and Technology, Pt. I and II, J. H. Saunders and K. C.

Frisch, Interscience Publisher, 1964.15. Developments in Polyurethanes, J. M. Buist, Applied Science Publishers, London,

1978.16. Thermoplastic Polyurethane Elastomers, C. S. Schollenberger, in Handbook of Elas-

tomers, A. K. Bhowmik and H. L. Stephens, Eds., Marcel Dekker, New York, 1988,pp. 375–407.

17. R. Heath, Journal of Coated Fabrics, vol. 15, Oct., 1985, pp. 78–88.18. J. W. Rosthauser and K. Nachtkamp, Journal of Coated Fabrics, vol. 16, July, 1986,

pp. 39–79.19. J. T. Zermani, Journal of Coated Fabrics, vol. 14, April, 1985, pp. 260–271.20. J. T. Tsirovasiles and A. S. Tyskwicz, Journal of Coated Fabrics, vol. 16, Oct., 1986,

pp. 114–121.21. J. Goldsmith, Journal of Coated Fabrics, vol. 18, July, 1988, pp. 12–25.22. F. B. Walter, Journal of Coated Fabrics, vol. 7, April, 1978, pp. 293–307.23. H. G. Schmelzer, Journal of Coated Fabrics, vol. 17, Jan., 1988, pp. 167–181.24. R. W. Oertel and R. P. Brentin, Journal of Coated Fabrics, vol. 22, Oct., 1992, pp.

150–159.25. J. R. Damewood, Journal of Coated Fabrics, vol. 10, Oct., 1980, pp. 136–150.26. Polyurethane structural adhesives, B. H. Edwards, in Structural Adhesives Chem-

istry and Technology, S. R. Hartshorn, Ed., Plenum, New York, 1986, pp. 181–215.27. C. S. Schollenberger and F. D. Stewart, Advances in Urethane Science and Tech-

nology, vol. 2, 1973, pp. 71–108.28. Surface Analysis and Pretreatment of Plastics and Metals, D. M Brewis, Macmillan,

New York, 1982.29. N. K. Porter, Journal of Coated Fabrics, vol. 21, April, 1992, pp. 230–239.30. Textile reinforcement of Elastomer, W. C. Wake and D. B. Wootton, Applied Science

Publishers Ltd., London, 1982 (and references therein).31. A. G. Buswell and T. J. Meyrick, Rubber Industry, Aug., 1975, pp. 146–151.32. B. T. Knight, Journal of Coated Fabrics, vol. 21, April, 1992, pp. 260–267.33. Coated fabrics, B. Dutta, in Rubber Products Manufacturing Technology, A.

Bhowmik, M. M. Hall and H. A. Benary, Eds., Marcel Dekker, New York, 1994,pp. 473–501.

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34. Vulcanization and curing techniques, A. K. Bhowmik and D. Mangaraj, in RubberProducts Manufacturing Technology, A. K Bhowmik, M. M. Hall and H. A. Benary,Eds., Marcel Dekker, New York, 1994, pp. 315–394.

35. Radiation cured coatings, V. Koleske, in Coating Technology Handbook, D. Satas,Ed., Marcel Dekker, New York, 1991, p. 659.

36. C. Decker, Journal of Coating Technology, vol. 59, no. 751, Aug., 1987, pp. 97–106.37. E. Krijnen, M. Marsman and R. Holweg, Journal of Coated Fabrics, vol. 24, Oct.,

1994, pp. 152–161.38. C. Bluestein, Journal of Coated Fabrics, vol. 25, Oct., 1995, pp. 128–136.39. W. K. Walsh, K. Hemchandra and B. S. Gupta, Journal of Coated Fabrics, vol. 8,

July, 1978, pp. 30–35.40. B. S. Gupta, K. Hemchandra and W. K. Walsh, Journal of Coated Fabrics, vol. 8,

Oct., 1978, pp. 183–196.41. B. S. Gupta, W. S. McPeters and W. K. Walsh, Journal of Coated Fabrics, vol. 9,

July, 1979, pp. 12–24.42. W. Oraby and W. K. Walsh, Journal of Applied Polymer Science, vol. 23, 1979, pp.

3227–3242.43. W. Oraby and W. K. Walsh, Journal of Applied Polymer Science, vol. 23, 1979, pp.

3243–3254.44. A. H. Luiken, M. P. W. Marsman and R. B. M. Holweg, Journal of Coated Fabrics,

vol. 21, 1992, pp. 268–300.

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Textile Substrate for Coated Fabric1


Awide range of textile materials is used as substrates for coated fabrics.These may be woven, knitted, or nonwoven materials. The importance of

textile materials can be gauged from the use of several billion square meters offabric every year.

The types of fiber commonly used in coating are cotton, rayon, nylon,polyester, and blends of polyester with cotton or rayon, depending on the enduse requirements. Polyester is the most popular in staple form for nonwovenmaterial and in spun form for woven material. Polypropylene is emerging as thefiber of choice because of its low specific gravity, strength properties, chemi-cally inert nature, and low cost. However, its poor dyeability, adhesion, andthermal stability are disadvantages that need to be overcome. High performancefibers like Kevlar®, Nomex®, PBI, etc., are used in specialized applications.

In woven form, plain, basket, twill, and sateen constructions are generallyused. Among the knitted constructions, circular knits are used as a substratefor upholstery fabric. Warp knit fabrics, particularly weft inserted warp knits(WIWK), are preferred for making coated fabrics for special applications. Non-woven fabrics, produced by different techniques, find use in sanitary and medi-cal products, apparel, artificial leather, dot-coated fabrics for fusible interlinings,etc.

The emerging trends in the use of textiles can be summarized as follows [1]:

(1) Development of polyester fiber with lower elongation or higher modulus,higher adhesion, and microdenier filament for greater cloth cover/surfacearea

(2) Greater use of polypropylene

1This chapter was contributed by N. Kasturia, R. Indushekhar, and M. S. Subhalakshmi, DMSRDE,Kanpur, India.

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(3) Use of longer roll length and wider fabric to lower the cost

(4) More use of textured, Dref, and core spun yarns for improved adhesion

(5) Greater use of nonwoven and WIWK

The choice of proper fabric for coating is as important as the selection of thepolymer, because it offers the primary physical property to the end product. Forproper selection of fabric, the following aspects need to be considered:

� strength and modulus� creep behavior� resistance to acids and chemicals� adhesion requirement� resistance to microbiological attack� environment of use� durability� dimensional stability� cost

The following characteristics should be considered when designing a textilesubstrate to meet specific end use requirements:

(1) Fiber type and form such as staple, filament, etc.

(2) Yarn type and construction

(3) Fabric form, i.e., woven, nonwoven, and knitted and their construction


The textile fabric/substrate used for coating is made of textile fibers. Thereare two main types of fibers: natural fibers and man-made or synthetic fibers.The natural fibers may be of vegetable origin, such as cotton, kapok, flax,coir, sisal, etc.; of animal origin such as wool, silk, etc.; and of mineral ori-gin, such as asbestos. The vegetable fibers are cellulosic in nature, the animalfibers are proteins, and asbestos is a silicate. The organic man-made fibers areessentially of two types: derived from cellulose, such as rayon and acetate,and synthetic polymers, such as nylon, polyester, acrylics, polypropylene, etc.Metallic fibers and glass fibers are inorganic man-made fibers. The properties ofsome important fibers used in the coating industry are discussed in this section.The physical and chemical properties of the fibers have been summarized inTables 2.1 and 2.2, respectively.

2.2.1 COTTON

It is known as the king of fibers. Cotton is a cellulosic (∼94% cellulose)staple fiber. The fiber length varies from 10–65 mm, and fiber diameter ranges

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TABLE 2.1. Important Physical Properties of Fibers.

Fibers Cotton Rayon Nylon Polyester Polypropylene Aramid(Nomex®)

Properties Viscose Acetate Normal Normal High Highapparel

IndustrialStaple tenacity

Multifilament Stapletenacity


Fiber fiberfiber fiber


Specific gravity 1.52−1.55 1.52 1.32 1.14 1.14 1.36 1.36 0.90 0.90 0.90 1.38

Tensile strength g/d 3−5 2.6 1.4 4.1−5.5 6.3−8.18 3.5 9.5 5−7 4−6 5.5−8.5 5.3

Elongation atbreak %

4−13 10−30 25−50 26−32 14−22 10−40 −− 15−35 20−35 15−25 22

Moisture regain %at 21◦C, 65% RH

8.5 ∼=13 6.3−6.5 4 4 0.4 0.4 negligible negligible negligible 5−5.2

Effect of heat(a) Resistant 150◦C 150◦C 180◦C 180◦C −− 370◦C

temperature(b) Decomposition

temperature230◦C 210◦C −− −− −− ∼500◦C

(c) Melting Decomposes Decomposes 250◦C (Nylon 66) 250◦C 160--175◦C Decomposestemperature 215◦C (Nylon 6)

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TABLE 2.2. Important Chemical Properties of Fibers.

Fibers Cotton Rayon Nylon Polyester PolypropyleneAramid



Effect ofsunlight andatmosphere

Loss of tensilestrength anddiscolorationof fibers occur

Loss of tensilestrength

Appreciabledegradationby sunlight

Low degradation inshade. Directsunlight weakensfibers

Rapiddegradationto sunlight andweathering.

Resistance toaging is excellent

Effect ofmicroorganism

Mildew,microorganismsdegrade the fiber

More resistantthan cotton

Resistant Resistant Resistant Resistant

Effect of acids Deterioratesthe fiber.Mineral acidsdegrade morereadily thanorganic acids

Same as cotton Affected byconcentratedmineral andorganic acids

Resistant to mostmineral acids.Concentratedsulfuric aciddecomposes fiber

Excellentresistance toacids

Not significantlyaffected, butis attacked byboiling sulfuricacid

Effect of alkalis Resistant at roomtemperaturebut swellingoccurs

Same as cotton Virtually noeffect

Resistant to alkali atroom temperaturebut hydrolyticdegradationoccurs at boilingtemperature

Resistant toalkalis

Resistant to alkalis

Effect ofsolvents/oxidizingagents

Resistant tocommonhydrocarbonsolvents.Oxidizingagentsconvert it tooxycellulose

Same as cotton Benzene,chloroform,acetone, andether do notaffect, but itdissolves inphenols andstrong acids

Resistant tohydrocarbonsolvents. Solublein m-cresol,o-chlorophenol athigh temperature

Insoluble inorganicsolventsat roomtemperature.Dissolves inhot decalin,tetralin. Attackedby oxidizing agents

Resistant to mostorganic solvents

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from 11–22 µm, respectively. The fiber has good strength due to the largenumber of interchain hydrogen bonds present in the polymer chain. Cotton isa natural fiber with wide variation in properties. This variation is caused bydifferences in climatic conditions in the regions where the cotton is grown.A good quality cotton fiber is characterized by its finer fiber diameter andlonger staple length. The important commercial varieties are (1) Sea Island,(2) Egyptian, (3) American upland, and (4) Indian cotton. Sea Island and Egyp-tian cotton have higher staple length and produce finer quality yarns. Indiancotton has shorter fiber length and produces coarser yarns. American uplandcotton lies between these two categories for quality and fiber length.

Cotton has moderate mechanical strength when dry but good wet strength.The resiliency of the fiber is low, therefore, cotton fabrics wrinkle easily. Due tohigh moisture absorption of the fiber, cotton fabrics are comfortable as summerwear. Cotton is extensively used for apparel fabrics as well as industrial textileslike canvas, ducks, etc. The fabric has excellent adhesion to coated/laminatedpolymeric film.

2.2.2 RAYON

Rayons are man-made fibers derived from cellulose. Viscose rayon is regener-ated cellulose, while acetate rayon is obtained by acetylation of cellulose. Boththe fibers are characterized by high luster and are considered as artificial silk.Viscose is obtained by treating wood pulp with caustic soda solution to formsoda cellulose. It is then treated with carbon disulfide to form cellulose xan-thate solution. The alkaline cellulose-xanthate, is ripened, and on achievingthe required viscosity, the solution is spun into a coagulating bath of dilute(10%) sulfuric acid. The viscose filament gets precipitated there. For makingacetate rayon, wood pulp or cotton linters are treated with a solution of aceticanhydride in glacial acetic acid to form secondary cellulose acetate, which hasfiber-forming properties. The secondary cellulose acetate is made into a dopewith acetone. The dope is forced through holes of a spinnerette, and the filamentis solidified by evaporation of acetone in hot air.

Like cotton, rayons are cellulosic in nature, as such, their chemical andphysical properties are similar to those of cotton. It is used in blends withpolyester for apparel fabrics, household textiles like furnishings and carpets,and in medical fabrics.

2.2.3 NYLONS

Nylon is the common name of linear aliphatic polyamides. The most impor-tant fibers in this class are nylon 66 and nylon 6. Nylon 66 is polyhexamethy-lene adipamide, a condensation polymer of hexamethylene diamine and adipicacid. The suffix 66 stands for the number of carbon atoms in the monomers.

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Nylon 6 is polycaprolactamide, the monomer being ε-caprolactam. The reactionsequences are given below.

nH2N(CH2)6NH 2 + nHOOC(CH2)4COOH 2)6-NH-CO-(CH2)4-CO]n- Hexamethylene diamine Adipic acid Nylon 66

- [NH(CH2 )5-CO-]n- ε-Caprolactam Nylon 6


_ _ _NH CO


Nylon is a group of synthetic super polymers, with much higher strengthand elongation than cellulosic fibers. It is available as regular translucent finefilament and can be converted into staple fibers.

Being a thermoplastic material, nylon fabric undergoes thermal shrinkage,and besides, it generates static electricity on friction. Special precautions there-fore have to be taken while processing nylon fibers. Nylon fabrics are widelyused for carpets, upholstery, and apparel. The high strength, elasticity, and abra-sion resistance enables nylon to be used for a variety of industrial end uses suchas filter fabrics, nets, webbings, cordages, parachutes, ropes, ballistic fabrics,etc.


Polyester refers to a class of polymers containing a number of repeat es-ter groups in the polymeric chain. Commercially available polyester fiber ispolyethylene terephthalate. It is known in different countries by different brandnames. In the U.K., it is known as Terylene, and in the U.S., it is known asDacron. The fiber is available in filament as well as in staple fiber form. Anumber of other polyesters have been converted into fibers, but they have notbeen exploited commercially.

+ nHO-CH2-CH2 OH

Dimethyl terephthalate Ethylene glycol Polyethylene terephthalate



Like nylon, polyester fabrics also generate static electricity and undergothermal shrinkage. Fabrics show poor adhesion to the coated polymeric film.

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The major use of polyester and its blends with cotton, rayon, and wool are inapparel fabrics, household fabrics, and industrial textiles.


Polypropylene is a hydrocarbon fiber, the properties are dependent on the mi-crostructure of the fiber. From the textile point of view, only isotactic polypropy-lene can be fibrillated, and the isotacticity index should be higher than 90%. Theaverage molecular weight of polypropylene fiber ranges from 100,000–300,000.Polypropylene fibers are produced in different forms like staple, monofilament,and multifilament.

Due to its light weight, negligible water absorption, and high abrasion re-sistance, polypropylene is widely used for making ropes, fishing nets, tuftedcarpets, etc.


These are aromatic polyamides that are closely related to nylons. In aramids,the aliphatic carbon chain is replaced by aromatic groups, bringing about consid-erable change in the properties of the resultant fiber. The first fiber introducedin this class by Dupont U.S.A. was Nomex®, which is chemically poly-m-phenylene isophthalamide, a condensation product of m-phenylene diamineand isophthalic acid. Nomex® is flame resistant and is widely used for fire-proof clothing. The p isomer, viz., polyparaphenylene terephthalamide (PPT)is known as Kevlar® fiber which possesses ultrahigh strength and modulus. Theproperties of Nomex® are given in Tables 2.1 and 2.2.

Aramids are high strength and high modulus fibers. They are mainly usedin composite reinforcement for ballistic protection, ropes, cables, and for fire-resistant clothing.


Spinning refers to the process of conversion of small fibers into yarns, or incase of synthetic fibers, spinning refers to the processes that convert polymersinto filaments. Most of the natural fibers like cotton, wool, etc., are availableonly as staple fibers having different fiber lengths. Spinning of natural fibers isdivided into the following systems depending upon the fiber lengths:

a. Short staple spinning system or cotton spinning system

b. Long staple spinning system or wool spinning system

Synthetic filaments, when converted to staple fibers, are spun by a processsimilar to that of cotton or wool.

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Cotton, which is a vegetable fiber, undergoes a sequence of processes beforebeing spun into yarn. This sequence includes ginning, opening and cleaning(blow room), carding, lap formation (sliver and ribbon lap), combing (optional),drawing, roving, and, finally, spinning. The flowchart (Figure 2.1) gives anoverview of the sequential operations involved in cotton spinning.

Figure 2.1 Spinning of cotton.

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Ginning is the starting process to which cotton is subjected on its way fromthe field to the textile mill before it is spun into yarn. Ginning is the process ofseparating cottonseed from the fiber. During this process, foreign matter likeleaf bits, stalks, hulls, etc., are removed. Care is taken to preserve the qualityof fiber, particularly the fiber length. The cotton fibers removed from the seedare compressed into large bales and sent to mills for processing. Blow Room

In a textile mill, this is the first preparatory process. In the blow room, cot-ton bales are subjected to mixing, opening, and cleaning processes. The cottonbales from different farm fields are fed into a mixing bale opener (MBO). Mix-ing of different varieties of cotton is done to improve the uniformity, therebyimproving the quality and minimizing the raw material cost. In some cases,cotton fibers are blended with other fibers to manufacture special yarns withdesired properties. The MBO opens the compressed cotton and mixes the differ-ent varieties of cotton by rotating cylindrical beaters. Thus, the closely packedfibers are loosened, and during this process, dirt and other heavy impurities areseparated from the fiber, either by gravity or by centrifugal force. The loosenedfibers are converted into a lap of smaller tufts called flocks. Carding

The fibers are received at the carding machines either in lap form or as flocks.Lap and flock feeding have their own advantages and disadvantages. However,most modern mills have flock feeding systems. The main objectives of thecarding process are to continue the cleaning process, removing some amount ofshort fibers, fiber individualization, partially aligning the fibers in the directionof the fiber axis, and disentangling neps (small entangled collection of immaturefibers). In the carding zone, the fibers pass over the main cylinder. The maincylinder is made of cast iron and is 120–130 cm diameter. This cylinder iscovered with fine sawtooth wires. On the top of the cylinder, there are a numberof moving flats that are joined to form an endless, circulating band. The flats arecast iron bars with one side clamped with a clothing strip, which is rubberizedfabric fixed with angled steel wires. These flats and main cylinder togetherform the main carding zone. The carding action involves the transfer of fibersfrom the cylinder surface to the flat surface and vice versa. During the multipletransfer, the wire points in the cylinder try to retain the fiber, and at the sametime, the wire points in flats try to pluck the fiber. But, most of the fibers areretained in the cylinder wire points, as the flats rotate at a much slower speedthan the main cylinder. This process leads to fiber individualization.

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The carding process opens up the collected mass of fibers so that the fibersbecome individual. However, the fibers in the card sliver are not completelyaligned or oriented in the fiber axis. Some fibers lie haphazardly in the sliver.Thus, the card sliver is given a minimum of two drafting processes before itgoes to the next machine. In this process, the sliver is passed between sets ofrollers that are running at different speeds, each succeeding pair rotating fasterthan the previous so that the fibers are pulled in a lengthwise direction. Thesetwo drafting operations are achieved by the sliver lap and ribbon lap machines.To improve the uniformity of the sliver, it is subjected to the process calleddoubling: Doubling is the process of combining a number of slivers. By thisprocess, the thin and thick places present in the sliver are evened out. In the sliverlap machine, 16–20 card slivers are creeled and passed through the feed table tothree pairs of drafting rollers for the drafting operation. The drafted slivers arethen taken to two pairs of calender rollers that compress the sliver material. Thisdrafted and compressed sliver material called lap is wound finally on a spool. Combing

Combing is an optional process, which is introduced into the spinning of finerand high quality yarns from finer cotton. For coarser cotton fibers, the combingoperation is usually omitted. This is the process of removal of a predeterminedlength of short fibers present in the fiber assembly, because the presence ofshort fibers reduces the yarn quality by increasing the number of thin and thickplaces, neps, and hairiness, and also lowers the tenacity. The presence of shortfibers and the inappropriate configuration of the fibers in the drawn sliver wouldnot allow drafting and the ring frame operations to be effective. Thus, combingis an important process next to carding for spinning fine yarns.

In the combing operation, lap from the lap roller is unwound and fed to thenippers by the feed rollers periodically. The fiber material is gripped betweenthe top and bottom nippers that keep the material ready for rotary combing.The rotary comber is a cylindrical device having needles fitted in a part of itssurface. The comber needles enter the fringe and comb and straighten the fibers.During this operation, short fibers, which are not under the grip of the nippers,are combed away with the needles. Drafted slivers are finally delivered into thecan. In a comber, there are eight feeding heads. In each head, one lap is fed,and the comber output is in the form of a sliver. Drawing

Sliver is taken from combing machine to the drawing machine. The mainobjectives of the drawing process are to further straighten the fibers, make them

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parallel to the fiber or sliver axis, and improve the uniformity by doubling.Blending of two different fibers like polyester and cotton, polyester and viscose,etc., is also carried out in draw frame in the case of manufacturing blendedyarn. Fiber straightening is achieved by drafting the slivers. During the draftingprocess, the linear density (weight per unit length) of sliver is also reduced. Theoperating principle of draw frame is that four to eight card/comber slivers arefed to the drafting arrangement through feed rollers, which are carried in a creelframe or table. The drafted slivers come out as an even web that is immediatelycondensed into a sliver to avoid disintegration of the web by a convergingtube. Speed Frame/Fly Frame

The uniform sliver obtained from the draw frame, subsequently goes to thespeed frame, which is the final machine in the spinning preparatory operations.The main tasks here are attenuation of fibers and formation of a suitable in-termediate package. Attenuation is the reduction in the linear density of thesliver. The extent of reduction is such that it is suitable for spinning into a yarn.The attenuation of the sliver is achieved by drafting. By this drafting opera-tion, the sliver becomes finer and finer, and the resultant product is called the“roving.” After the drafting operation, the roving is wound on the bobbin. Dur-ing winding, a little amount of twist is imparted to the roving. Ring Spinning

The final process of yarn formation, i.e., spinning, is carried out in the ma-chine called a ring frame. In this process, the roving is attenuated into yarn bydrafting. Substantial amount of twist is inserted to the yarn, then it is woundon a bobbin. In other words, drafting, twisting, and winding are the steps tak-ing place during the spinning operation. In the spinning process, the roving,which is several times thicker than the yarn, is subjected to a higher amount ofdraft when it passes through three pairs of closely associated rollers moving atdifferent surface speeds. For attenuation, the yarn delivery rollers revolve at ahigher speed than the feed rollers.

The yarn produced from the spinning machine is a single yarn. However, asper the end use requirement, it may be twisted together with two or more singleyarns in the doubling machine to achieve stronger and more uniform yarn. Newer Methods of Spinning

Over the centuries, many ways have been devised for conversion of fibersinto yarns, but in the past thirty years, the search for a new and more econo-mical spinning system has been actively pursued in many parts of the world. A

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new spinning system popularly known as rotor spinning or open-end spinningwas introduced in the late 1960s. It has made great impact on the textile in-dustry, especially in terms of rate of production. Other spinning processes, likeelectrostatic spinning, air-vortex spinning, friction spinning, and disc spinning,are also different types of open-end spinning processes. The twist spinning,self-twist spinning, wrap spinning, false twist spinning, and adhesive processesare only of academic interest and have not become popular. All of these newspinning systems produce yarns having quality that differs to a certain extentfrom that produced by the more traditional ring-spinning process.


Synthetic fiber spinning is entirely different from staple fiber spinning whichwas discussed earlier. In synthetic fiber spinning, the fiber/filament is madeby extruding the polymer liquid through fine holes. In synthetic fiber spinning,the diameter of the filament is determined by three factors, i.e., the rate at whichthe dope is pumped through the spinnerette, the diameter of the spinneretteholes, and the rate at which they are taken up by the take-up rollers.

Synthetic fiber spinning is divided into three systems based on the meltabilityand solubility of the polymer. They are melt spinning and solution spinning.Solution spinning may be further divided into two systems on the basis of natureof the solvent: dry spinning and wet spinning. Melt Spinning

Polymers that melt on heating without undergoing any decomposition arespun by a melt spinning system. In this system of spinning, the polymer chipsare fed into a hopper. From the hopper, the chips are passed to a spinningvessel through a pipe. In the spinning vessel, the polymer chips fall onto anelectrically heated grid that melts the chips and has a mesh too small to passthe chips until they are melted. The molten polymer then passes into the pooland filtering unit. The filtering unit consists of several layers of metal gaugeand sand kept alternately with coarse, fine, and very fine mesh and particle size,respectively. This filtering unit filters any impurities out of the molten polymermass, as they may block the fine holes present in the spinnerette plate. Afterpassing out of the filtering unit, the polymer is forced through the holes in thespinnerette plate and emerges from the plate. As the filaments emerge, they aredrawn away from the outlet, stretching the polymer before it cools. Immediatelyafter emerging, cool air is passed to solidify the melt. The solidified melt, nowcalled filament, is then passed through a spin finish bath, an antistatic agent isadded, and the filament is wound onto a bobbin by a winder. The filament is thenused as such or is imparted crimped or cut into staple fibers. Polyester, nylon,and polypropylene polymers are converted into filaments by this technique.

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Some polymers, e.g., acrylics, undergo decomposition on heating. Therefore,they cannot be spun by the melt spinning system. These polymers are spun by asolution spinning system. In this system, the polymer is dissolved in a suitablesolvent, and the polymer solution, generally called dope, is extruded throughthe holes of the spinnerette for making filaments. If the solvent selected is avolatile solvent, then the polymer is spun by a dry spinning system; if it is anonvolatile solvent, the polymer is spun by a wet spinning system. Polyacrylonitrile (PAN) polymer (acrylic fiber) can be spun by either a dry or wet spinningsystem.

In a dry spinning system, the polymer is dissolved in a suitable volatile solventand forms a solution called dope. This dope is fed to the spinning head from afeed tank through pipes. A metering pump controls the constant and uniformflow through the spinnerette. The extruded stream of solution flows out into ahot air chamber. On evaporation of the solvent by hot air, the solidified polymerfilament is drawn, taken up through a spin finish bath, and wound. The solventmay be recovered from air by adsorption on active carbon.

In wet spinning, the polymer is dissolved in a nonvolatile solvent, and thepolymer solution is regularly fed to the filter and spinning head. The spin-nerette is submerged in a coagulation bath, and as the polymer emerges out ofthe spinnerette, the polymer in the solution is precipitated by the bath liquidand soldifies in filaments. The filament is then wound onto the bobbin after spinfinish application.


The process of converting a set of yarns into a fabric, on a loom, is calledweaving. The mechanism of interlacing two sets of yarns at right angles toeach other, according to a desired design, is done on the loom. Woven fabricsare more widely used in apparel and industrial applications. The two sets ofyarns, warp (longitudinal thread), and weft (lateral thread) require a separateset of processing before they are ready to be woven on the loom. This becomespertinent, especially if one is looking for special properties like rib effect,absorbancy, and adhesion properties of the fabric. The properties of a gray fabric(fabric coming out from a loom) depend on fiber properties, yarn properties,density of yarns in the fabric, weave, and yarn crimp.


To produce a fabric on any loom, the five operations given below are neces-sary. The first three operations are generally termed fundamental operations.

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(1) Shedding: the separation of warp threads (longitudinal) into two layers, oneset of which is lifted and the other which is lowered to form a space sufficientenough to send a shuttle of weft yarn (lateral yarn) for interlacement. Eachspecific set of warp yarns is raised by means of a harness or heald frame.The design of the weave depends on the sequence of raising of the set ofyarns forming the shed during insertion of the filling yarn. Tappets, cams,dobby, and Jacquard mechanisms are used as shedding devices.

Tappet and cams can handle up to fourteen different harnesses and arewidely used for simple fabrics. Dobby is a shedding device placed on topof the loom that can handle up to forty harnesses and is used for producingsmall, figured patterns. Jacquard device is also placed on the top of theloom and can handle individual warp yarns. This enables the weaving ofcomplicated and elaborate designs.

(2) Picking: the insertion of weft yarn by passing from one end of the fabricto the other end through the shed created due to parting of warp yarns intoupper and lower layers. Generally, shuttles, projectiles, rapiers, etc., areused as vehicles for the transfer of weft.

(3) Beating up: pushing the newly inserted weft (pick) into the already wovenfabric to the end point (fell) is known as the beating process. The beatingforce employed has a significant influence on the closeness of the fabric.

(4) Warp let off: delivering the series of warp threads simultaneously at arequired rate at a suitable constant tension is termed as warp let off. Therate of releasing the warp threads in conjunction with the take up of clothdecides the pick density in the fabric.

(5) Cloth take up: moving the fabric from the formation zone at a constant rateand winding the fabric onto a roller is called cloth take up. By controllingthe warp tension, the let off motion decides the crimp in the threads.


The fundamental weaves are plain, twill, and satin weaves. These are thebasic weaves from which many new kinds of weaves are derived. The smal-lest unit of design that appears repeatedly in a weave pattern is called therepeat. The weaves that are generally used in the coating industry are discussedbelow. Plain Weave

Plain weave is the simplest form of interlacing two sets of yarns. The yarnsinterlace each other at right angles in alternate order. It has the smallest num-ber of yarns in the repeat, which is two. The maximum possible number of

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intersections of warp and weft yarns makes a plain weave fabric the strongestand stiffest among the various woven structures. About 40% of all fabricsproduced are in plain weave. Some examples of plain weave fabrics are voile,muslin sheeting, mulmul, poplin, cambric, lawn, organdy, shantung, taffeta,canvas, etc. Apart from the plain weave, derivatives of plain weave (weaveconstruction based on plain weaves) are widely used in various industrial fab-rics, e.g., tents/shelters, protective clothing, parachutes, and other specializedclothing.

The derivatives of plain weave are as follows:

(1) Basket weave: this is a variation of the plain weave that uses two or morewarp yarns simultaneously interlaced with two or more fillings, giving abalanced structure to produce a design that resembles the familiar patternof a basket. They are woven in a pattern of 2 × 2, 3 × 3, or 4 × 4 withtwo or more filling yarns interlaced with a corresponding number of warpyarns.

(2) Oxford weave: it varies slightly from the regular basket weave in that it has2 × 1 construction, i.e., one filling yarn passes alternately over and undertwo warp yarns that act as one thread. Generally, the fineness of the weftyarn is approximately equivalent to the fineness of the warp yarns.

The basket/mat weave consists of a fewer number of interlacings per cmcompared to plain weave, and hence, it allows more threads to be inserted percm. For this reason, the cloth cover of basket weave is high compared to basicplain weave, but due to fewer intersections/cm, this fabric is more flexible anddrapes (hangs) well. In applications where tear strength is important, basketweaves are preferred to plain weave. Twill Weaves

In twill weave, the first warp yarn interlaces with the first weft yarn, thesecond warp yarn with the second weft yarn, the third warp yarn with the thirdweft yarn, and so on up to the end of the repeat. Owing to this order of warpand weft yarns interlacing, fabrics with a twill weave pattern exhibit a diagonalstripe directed at an angle of 45◦ (diagonal lines) from the left upward to theright. These weaves are employed for the purpose of ornamentation and tomake the cloth heavier and have better draping quality than that which can beproduced with the same yarns in a plain weave. Twill lines are formed on bothsides of the cloth. The direction of diagonal lines on the face side of cloth isopposite to that on the back side, coinciding, respectively, with the weft andwarp floats on the other side. Thus, if the warp floats predominate on one sideof the cloth, weft floats will predominate on the other side of the cloth. The

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twill weave fabrics include canton flannel, covert cloth, denim, drill, gabardine,jean, khaki, whipcord, etc. Satin and Sateen Weaves

In a satin weave, the warp skips a number of weft yarns before interlacement,thus, warp yarn dominates the face of the fabric. On the other hand, in a sateenweave, the weft yarn skips a number of warps prior to interlacement, and weftdominates the fabric face. For example, when a warp skips seven fillings beforeit interlaces, the weave is termed an eight float satin. Because of the longyarn floats, satin and sateen fabrics reflect more light and impart high glossto the surface. These fabrics are also characterized by a maximum degree ofsmoothness. Satin weave fabrics drape well because the weave is heavier thanthe twill weave, which in turn, is heavier than the plain weave. Some satinweave fabrics include antique satin, bridal satin, cotton satin, etc.

Each weave can be presented in a square paper design (point paper design) toillustrate weave pattern. Vertical columns of squares on a point paper representwarp ends, and horizontal rows of squares represent picks. A marked squareon the point paper indicates that the warp end is raised above the pick, while ablank square means that the warp end is lowered under the pick during weaving.Figure 2.2 shows the graphic symbol of some basic weaves.

2.4.3 LOOMS

All woven cloth is made on some sort of loom. The conventional looms areshuttle looms. The shuttle that carries the yarn through the shed of warps is awooden boat-like container about 30 cm long that carries a bobbin called pirnonto which filling yarn is wound. The shuttle is propelled through the shedformed by warps carrying the filling yarn across the width of the material as itunwinds from the bobbin. The conventional shuttle looms consume a great dealof power and are relatively slow in operation, noisy, and not satisfactory for widefabrics. Because of these drawbacks, shuttleless looms have been developed.In these looms, the yarn is carried directly from a large cone of yarn locatedoutside of the loom. The operations of some important shuttleless looms arediscussed below.

(1) Projectile looms: the picking action is accomplished by a series of smallbullet-like projectiles that grip the filling yarn and carry it through the shedand then return empty. These looms have speeds between 300–600 picksper minute (ppm) depending on the width of the fabric.

(2) Rapier looms: instead of the projectile, a rapier-like rod or steel tape isused in these looms to carry the filling yarn. More commonly, two rods are

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Figure 2.2 Graphic symbol of some basic weaves: (a) plain weave, (b) 2 × 2 basket weave,(c) 2 × 1 basket/oxford weave, (d) 2 × 2 twill weave, and (e) 4 × 1 satin weave.

used: one carries the yarn halfway, where the end of the yarn it carries istransferred to a rod propelled from the other side that pulls the yarn the restof the way, while the first rod retraces. This speeds the operation.

(3) Water-jet looms: in water-jet looms, a predetermined length of pick is car-ried across the loom by a jet of water propelled through the shed. Theselooms operate at high speeds of ∼600 ppm. These looms can produce su-perior quality fabrics.

(4) Air-jet looms: a jet of air is used to propel the filling yarn through the shedat a relatively high speed of ∼600 ppm.


Knitting is a process whereby fabrics are formed by the interlacing of neigh-boring yarn loops. The fabric manufactured by knitting has distinctly different

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properties than those of the woven structures. The knitted structure may beformed either (a) by weft knitting, in which one or more individual weft supplyyarns are laid across beds of needles so that loops of yarns are drawn throughpreviously made loops, or (b) by warp knitting, in which the fabric is formedby looping together parallel warp yarns as they are fed collectively from a warpbeam. In a knitted fabric, the yarn density is denoted by wales and courses percm. The wales are a series of loops in successive rows lying lengthwise in thefabric; they are formed by successive knitting cycles and the intermeshing ofeach new loop through the previously formed loop. It gives an indication ofneedles per cm in the machine. Courses are the horizontal ridges in the weftdirection that give an idea of the stitch length. A stitch is made when a loopof yarn is drawn through a previously made loop. Due to fewer manufacturingsteps, knitted fabrics are easy to produce compared to woven fabric. Moreover,the changeover from one structure to the other can be readily done. Knittedfabrics have the following important characteristics:

� high extensibility� shape retention on heat setting� crease and wrinkle resistance� pliability� better thermal insulation property� better comfort property

These fabrics find wide application in casual wear, sportswear, and under-garments. In the coating industry, they are widely used for making upholsteryfabric and leather cloth.


Weft knitting process is the method of creating a fabric via the interlockingof loops in a weftwise or crosswise direction. The three most popular andfundamental structures of weft-knitted structures are jersey (plain), rib, andpurl.

In jersey-knit fabrics, the vertical component of the loops appears on the faceside, and the horizontal component is seen on the reverse side of the fabric. Theface side of jersey usually has a softer hand than the reverse side. The fabric ischaracterized by a smooth, regular surface with visible wales on the face side,and a series of semicircular loops on the reverse side. The drawback to thesefabrics is that a cut fabric easily ravels in knitting and reverse directions. Jerseyfabrics can be made in circular or flat knitting machines with a set of needlesin circular or linear positions.

Rib structure differs from the jersey fabric in that it has identical appearance inboth directions. The rib fabric is produced when stitches intermesh in opposite

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directions on a walewise basis. When opposite interlocking occurs in everyother wale, the fabric is known as 1 × 1 rib. Similarly, when the interlockingoccurs at every three wales in one direction to every two wales in the oppositedirection, the fabric is termed as 3 × 2 rib. The rib fabric can be producedon a simple circular knitting or on a flat knitting machine using additionalattachments.

Purl fabrics are produced on machines with needles that have hooks at bothends. Purl structures have one or more wales that contain both face and reverseloops.


In warp knitting, each yarn is knitted by one needle. The needle bar thatcarries the needle moves sideways as well as up and down, so that the yarns arecarried vertically and, to a limited extent, diagonally. This diagonal motion isneeded to assure that the yarns interlace not only with the stitch directly belowbut also with stitches to the side. The fabric is formed by the intermeshing ofparallel warp yarns that are fed from a warp beam. Here, the warp yarns movein a zigzag motion along the length of the fabric which results in a loop at everychange of direction as individual yarn is intermeshed with neighboring yarns.Compared to weft-knit fabrics, warp-knit fabrics are flatter, closer, less elastic,and dimensionally more stable, as parallel rows of loops are interlocked in azigzag pattern. They have a higher production rate and can be produced in awider width. Warp-knit fabrics can be of different types of construction, i.e.,tricot, Raschel, simplex, and milanese. Among these, tricot and Raschel arecommonly used in industrial textiles.

In warp knitting, guide bars are used to guide sets of yarns to the needles.The pattern potential of the knitted fabric is controlled by these devices. Tricotfabrics are classified according to the number of guide bars used. Thus, one-bartricot uses one guide bar, and two-bar tricot uses two guide bars for production.Tricot fabrics are known for softness, wrinkle resistance, and drapability. Theypossess higher bursting and tear strength.

Compared to tricot warp-knit fabrics, Raschel fabrics are generally coarsegauge. However, the machines used for producing Raschel fabrics are moreversatile, and they have a very large pattern area for ornamentation. Typicalproducts made from this fabric include dressware, laces, powernets, swimwear,curtain nets, etc.

The majority of tricot fabrics are knitted from smooth filament yarns inlightweight construction. On the other hand, the Raschel machines are capableof producing heavier fabrics using spun yarns. Patterns of some important knitsare given in Figure 2.3.

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Figure 2.3 Patterns of some important knits: (a) jersey knit and (b) warp-knit tricot.


Nonwoven fabrics are constructed directly from a web of fibers withoutthe intermediate step of yarn manufacture as is necessary for woven, knitted,braided, or tufted fabric. These fabrics are extensively used in disposable andreusable goods because of their low cost and suitability for several special-ized applications, as in fusible interlinings, filter media, surgical wear, sanitarygoods, diapers, and wipers, etc. In the coating industry, they are widely usedin synthetic leather, poromorics, upholstery backing, and protective clothing.Nonwovens may be classified by the type of fiber used, method of web forma-tion, nature of bonding, and type of reinforcements used. The fibers commonlyused are cotton, nylon, polyester, rayon, acetate, olefins, and combinations.There are two distinct steps in the manufacture of nonwovens. The first step isthe manufacture of a web of fiber. The laid fibers, known also as a batt, do notpossess adequate strength. The second step involves entanglement or bondingof the fibers to develop adequate strength.


There are various methods for laying the web. In mechanical methods, com-pressed fibers are passed over rotating wire-covered cylinders (carding ma-chine). The wires pick up the fibers and deposit them in sheet or batt form. Asingle layer of the web produced from the card is too thin, as such, multiplelayers are often stacked to achieve the desired thickness. If the web layers are

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laid in parallel, they are known as parallel-laid web. These webs have higherstrength in the machine direction than in the cross direction. If the webs obtainedfrom the card are cross lapped (changing the orientation from direction of webtravel to cross direction), the ratio of strength between the machine direction tocross direction is reduced. Such webs are known as cross-laid webs.

An effective way to minimize the fiber alignment is to sweep the openedfiber coming out of a carding machine by a stream of air and then condensethe fiber on a slow moving screen or perforated drum. Such webs are knownas air-laid webs. Webs can also be produced by a wet process similar to theone used in the making of paper. In this process, the fibers are suspended inwater. The suspension is passed over a moving screen to remove the water.The remaining water is squeezed out of the web, and the web is dried. Websproduced by this method are denser than those produced by the air-laid process.The fiber alignment is also more random.

The spunbonded method is especially used for the manufacture of a nonwovenfrom continuous filament fibers. In this process, continuous filament extrudedthrough spinnerettes is allowed to fall through a stream of air on a movingconveyor. The desired orientation of the filaments in the web is achieved bycontrolling the stream of air, speed of conveyor, and rotation of the spinnerette.

If the fibers are thermoplastic, the batt can be thermally bonded by passagebetween the nip of the heated calender roll. Fusion of the fibers occurs at theintersections.


There are two types of bonding for the batt: entanglement of fibers andbonding by adhesives. Entanglement of Fibers

(1) Needle punching process: this is one of the most common mechanicalbonding processes. In this process, an array of barbed needles is pushedthrough the web. The barbs hold the fibers at the surface and push them intothe center, densifying the structure and leading to an increase in strength.The machine consists of a bed plate to support the web as the set of need-les penetrates the web and a stripper plate to strip the fabric off the needles.The number of penetrations per unit area controls the density, thickness,and permeability of the nonwoven fabric. Most needle-punched fabrics arereinforced by a scrim.

(2) Hydroentanglement: in this process, a fine jet of water is used to push fibersfrom the surface toward the interior of the batt. During impingement, theweb is supported on a bed. The force exerted by the jet is less than that

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exerted by needling, and as such, the hydroentangled structure is less denseand more flexible than that obtained by the needling process. The methodis useful for bonding relatively thin webs. Such fabrics are often termedspunlaced fabrics. Bonding by Adhesives

The bonding of fibers of the web can be achieved by using a variety ofadhesives, both in liquid and solid forms. Liquid adhesives can be solutions,emulsions, or pastes. They are applied on the web by dipping and squeezing,spraying, or kiss coating. After application of the adhesives, the solvent isevaporated, and the adhesive is cured by passage of the web through a heatedchamber using heated air or IR heaters.

Solid adhesives can be applied as hot melt by spraying or by using thegravure/rotary printing process and subsequently cooling the web. Solid pow-der adhesives can be applied on the web by scatter coating; the adhesive isactivated by passage through a heated chamber. If the web contains a mixtureof low melting fibers along with high melting or nonmelting fibers, e.g., rayonand polyester, passage of the web through a heated chamber or hot calenderrolls melts the thermoplastic fiber, leading to bonding. These are known asthermobonded fabrics.

The distribution of adhesive in the web is very important, because unlikein paper, movement of fibers in a nonwoven fabric is necessary to produce itstextile-like properties. Adhesives interfere with fiber movement. Small bondedareas separated by unbound areas constitute the textile property of the fabric. Ifthe adhesive fills the void between the fibers completely, the product becomessimilar to fiber-reinforced plastic.

The strength of a web can be enhanced by the incorporation of yarn in theweb. In the stitch-bonded process, the web is passed through a sewing or knittingmachine. The stitched structure holds the fibers of the web together. Thus, thefabric is an open-mesh yarn structure with interstices filled with nonwovenfibers. The technology is known as the “Arachne” stitch bonding process.


1. W. C. Smith, Journal of Coated Fabrics, vol. 15, Jan., 1986, pp. 180–197.


S. Adanur, Ed., Wellington Sears Handbook of Industrial Textiles, Technomic PublishingCo., Inc., Lancaster, PA, 1995.

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M. Grayson, Ed., Encyclopedia of Textiles Fibres and Non Woven Fabrics, John Wiley &Sons, New York, 1984.

A. J. Hall, Standard Handbook of Textiles, Newness-Butterworth, London, 1975.F. Happey, Ed., Contemporary Textile Engineering, Academic Press, London, 1982.M. Lavin, and Sello, S. B. Eds., Handbook of Fibre Science and Technology, Chemical

Processing of Fibres and Fabrics, Fundamentals and Preparation, Part A, vol. 1,Marcel Dekker, New York, 1983.

H. F. Mark, Atlas S. M., and Cerina, E. Eds., Man Made Fibres Science and Technology,vols. 2 and 3, Interscience Publishers, New York, 1968.

R. Mark, and Robinson, A. T. C., Priniciples of Weaving, The Textile Institute, Manch-ester, 1976.

R. W. Moncrieff, Man Made Fibres, John Wiley & Sons, New York, 1963.V. Mrstina and Fejgl, F., Needle Punching Textile Technology, Elsevier, New York,

1990.A. T. Purdy, Non Woven Textiles, Textile Progress, vol. 12, no. 4, Textile Institute, U.K.,

1983.W. Scott-Taggart, Cotton Spinning, Universal Publishing Co., Bombay, 1985.

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Coating Methods


COATING a layer of polymeric material on a textile imparts new character-istics to the base fabric. The resultant coated fabric may have functional

properties, such as resistance to soiling, penetration of fluids, etc., or have anentirely different aesthetic appeal, such as finished leather. There are variouscoating methods used to apply polymer to textiles. They can be classified onthe basis of equipment used, method of metering, and the form of the coatingmaterial. The various methods are given below.

(1) Fluid coating: the coating material is in the form of paste, solution, orlatices.

a. Knife coaters, wire wound bars, round bars, etc.: these are post-meteringdevices.

b. Roll coaters,reverse roll coaters,kiss coaters,gravure coaters,dip coaters,etc.: these are premetered application systems.

c. Impregnators: material to be coated is dipped in the fluid, and the excessis removed by squeeze roll or doctor blades.

d. Spray coaters: the material is sprayed directly on the web or onto a rollfor transfer.

(2) Coating with dry compound (solid powder or film):

a. Melt coating: extrusion coating, powder coating, etc.

b. Calendering: for thermoplastic polymers and rubber compounds,Zimmer process, etc.

c. Lamination

The choice of a coating method depends on several factors. They are asfollows:

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� nature of the substrate� form of the resin and viscosity of the coating fluid� end product and accuracy of coating desired� economics of the process


A fluid coating operation basically involves applying the coating fluid ontothe web and then solidifying the coating. There are common features in allcoating operations. The different modular sections of a coating machine areillustrated in Figure 3.1 and described below.

(1) Fabric let-off arrangement. Here, the base fabric is unwound and drawnthrough the machine under uniform tension. Many machines have accumu-lator sections, where the rolls are temporarily sewn together for continuousoperation, without interruption due to changeover of the rolls. A typicalaccumulator is shown in Figure 3.2.

(2) A coating head. It may be knife, roll, or any of the methods of fluid coating.

(3) Drying oven. All of the solvents are evaporated, and the film is solidified,dried, and cured. The oven may be steam heated, air heated (oil/forcedair), or electrically heated. For rubber-coated fabrics, vulcanization is car-ried out separately after removal of solvents by evaporation. For otherpolymers requiring higher temperature, drying and curing can be doneby IR heaters, gas-fired units, heater strips, etc. To prevent volatiles fromforming an explosive mixture, fresh air is continuously circulated through-out the oven. In the case of organosols, the drying rate is carefully con-trolled to prevent blister formation or cracking. To properly control solventevaporation, it is necessary to divide the oven into several zones, increas-ing the temperature of each zone in order to remove the solvent withoutblisters.

(4) Winding section. The fabric coming out of the oven is passed over coolingdrums to make it tack free. The fabric is then wound up in rolls.In addition, there is a drive unit that transports the substrate web through the

Figure 3.1 Layour of direct coating line: (1) fabric let-off arrangement, (2) coating head, (3) dryingoven, and (4) winding section. (Adapted with permission from G. R. Lomax, Textiles, no. 2.1992,c©Shirley Institute U.K. [1].)

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Figure 3.2 Line diagram of a fabric accumulator. Courtesy Sanjay Industrial Engineers, Mumbai,India.

coating head under constant tension. At times, the drive unit incorporatesa stenter frame to minimize shrinkage during the drying process. Coatingthickness can be measured by a β-ray gauge or from the web speed andflow rate of the coating fluid [1,2]. A general view of a coating plant isshown in Figure 3.3.

Figure 3.3 General view of a fluid coating plant. Courtesy Polytype, U.S.A.

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The coating process can be classified on the basis of stages of metering, i.e.,(a) process where the material is applied on the substrate and then metered and(b) process where the material is metered prior to application. A combination ofthese methods uses application of premetered excess followed by further meter-ing for more accurate coating. A discussion of the advantages and disadvantagesof the methods is presented below.

The first category, i.e., postmetering processes, is considered to be effectivefor coating noncritical weights on the substrate. As has been described earlier,in this class are the common knife coaters, wire wound (Mayer rod) coaters,single-roll squeeze coaters, etc. Here, excess coating is initially applied onthe textile substrate. After the substrate is wetted, a coating device meters thecoating to a predetermined thickness. The parameters necessary for consistencyin coating add-on are as follows:

a. Substrate tension

b. Viscosity of the coating material

c. Substrate uniformity and porosity

Any variation in these parameters may lead to a nonuniform coating. Coatingaccuracy is poor. The coating range is limited to about 0.02 to 0.2 mm thickness.However, the major advantage is their low investment cost and fast productchangeover.

In the second category, a premetered quantity of material is applied ontothe textile. The processes include roller coatings, gravure coatings, extrusioncoatings, and lamination. These methods are much more accurate and givehighly reproducible add on. The coating range is wider, 0.1 to 0.5 mm. However,the initial investment cost is higher [3].

The common methods of coating are described below.


Also known as spread coating, this is one of the oldest coating methods. Adry, smooth fabric is fed over the bearer roll under a knife known as a knifeor doctor blade. The coating material is poured in front of the knife by a ladleor by a pump over the entire width of the web. As the web is transportedunder the knife, the forward motion of the fabric and the fixed knife barriergive the viscous mass of the material a rotatory motion. This is known as therolling bank that functions as a reservoir of coating compound in front ofthe knife. To prevent the fluid from spilling over the edges of the fabric, twoadjustable guard plates known as dams are also provided. Proper tension is

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applied on the fabric as it is unwound, so that the fabric is taut under the knife.Most machines can coat fabric widths up to 1.5–2.0 m, but specially designedmachines can accommodate up to 4 m widths. Special care is taken that thematerial has adequate viscosity so that it does not strike through the fabric. Thecoated fabric then passes through the drying oven. The rate of evaporation ofthe solvent determines the rate of transport of the fabric, and thus, the coatingrate. The coating thickness is mainly controlled by the gap between the knifeand the web [4].


There are three distinct arrangements of knife coating. They are knife on air,knife on blanket, and knife on roll. These arrangements are given in Figure 3.4.

In the floating knife [Figure 3.4(a)] or knife-on-air coating, the knife is posi-tioned after a support table and rests directly on the fabric. In this arrangement,compressive force applied on the coating material is greater, and as such, thecoating compound enters the interstices of the fabric. This technique is usefulfor applying very thin, lightweight, impermeable coatings (as low as 7–8 g/m2)suitable for hot air balloons, anoraks, etc. [5].

Figure 3.4 Different types of knife coating: (1) support table, (2) rubber blanket, (3) rubber or steelroll, (4) knife, (5) web, and (6) coating material. (Adapted with permission from Encyclopedia ofChemical Technology, Vol. 6, 3rd Ed. 1979; and Encyclopedia of Polymer Science & Engineering,Vol. 3, 2nd Ed. 1985. Both c©John Wiley & Sons.)

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Web tension, viscosity, percent solids, and specific gravity of the coatingcompound play a significant role in the amount of coating deposited. Thehigher the viscosity of the compound, the greater will be its tendency to forcethe web away from the knife, resulting in a higher weight add on. On theother hand, it can be easily visualized that the coating weight will be lessif tension on the fabric is greater. The method is suitable for both closelywoven and open fabrics, because strike through does not affect the coatingoperation [6].

In the knife-on-blanket arrangement [Figure 3.4(b)], the web is supported bya short conveyor, in the form of an endless rubber blanket stretched between tworollers. Because the tension applied on the blanket results in a uniform pressurebetween the knife and the substrate, the fabric is not subjected to stretching inthis arrangement. It is possible to coat dimensionally unstable substrates withthis technique. The amount of coating is dependent on the tension of the blanket,which is adjusted by the rollers. Care should be taken that there is no damageto the blanket and that no foreign matter is adhered on the inside of the belt, asthis will result in an irregularity in coating weight.

The knife-on-roll system [Figure 3.4(c)] is the most important and widelyused technique for its simplicity and much higher accuracy. In this configuration,a suitably designed doctor blade is properly positioned on top of a high-precisionroller. The gap between the bottom of the blade and the thickness of the fabricthat passes over the roller controls primarily the coating weight. The roll maybe rubber covered or chromium-plated steel roll. The hardness of the rubber-covered roll may vary from 60 to 90 shore A, depending upon the type of fabric[4]. The advantage of a rubber-covered roll is that any fabric defects, such asknots and slubs having thickness greater than the fabric thickness, are absorbedby the roll surface, allowing free passage of the fabric through the coating knife.However, rubber rolls are not as precise as steel rolls and may cause variation inthe wet coating weight up to ±30 g/m2. Rubber rolls also have the disadvantageof swelling on prolonged contact with solvents and plasticizers. Steel rolls, onthe other hand, can give more precise coating [5]. The gap between the knifeand the roll can be adjusted by the screws provided on the mounting rod of theknife. In modern machines, the gap is controlled by pneumatic arrangement.Besides pneumatic, quick lifting is also provided to release lumps, splices, tornedges, etc. Materials with a wide viscosity range (up to 40,000 cps) can becoated by the knife-on-roll technique. It can also impart heavy coatings onfabric using a solventless system like plastisols. The coating method is bettersuited for dimensionally stable fabrics, which will not easily distort due totension applied on the fabric while pulling through the coater. Care is alwaystaken that the coating material does not strike through. In case of a strikethrough, steel rolls are easier to clean. A knife-on-roll coating plant is shown inFigure 3.5.

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Figure 3.5 A knife on roll coating plant. Courtesy Egan Davis Standard Corp., and U.S.A.


The profile of the coating knife and its positioning over the roll are importantparameters affecting coating weight and penetration. Numerous knife profilesare used in the trade, however, some common types are shown in Figure 3.6.

The knife profile [Figure 3.6(a)] is normally used for lightweight coating.The base of the knife may vary from 0.5 to 4 mm wide. The knife is cham-fered on the other side of the rolling bank. The sharper the base of the knife,the lower the coating weight. If the blade is chamfered on both sides, that isthe V-type profile [Figure 3.6(b)] a wedge effect is produced during coating,

Figure 3.6 Profiles of knives: (a) knife type, (b) V type, (c) bull nose, and (d) shoe. (Adapted withpermission from F. A. Woodruff. J. Coated Fabrics, Vol. 21, April 1992. c©Technomic PublishingCo., Inc. [5].)

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which puts considerable pressure on the coating material, resulting in muchgreater penetration of the material into the interstices of the fabric. This typeof profile is used where a high degree of penetration is required for good me-chanical adhesion. Multiple coats are applied to achieve the desired coatingweight of the end product, such as for tarpaulins, hoses, etc. A bull-nosed knife[Figure 3.6(c)] imparts heavy coating weights with little penetration into theweave and is suitable for easily damaged fabrics. The shoe [Figure 3.6(d)] knifeis so named because of its resemblance to a shoe. The front of the knife maybe straight or rounded. The base dimension may vary from 2–30 mm. The toeof the blade is nearest to the substrate. By varying the angle between the bladeand the roll, the elevation of the heel of the knife and the web can be altered.A wedge of coating compound is formed between the web and the heel ofthe knife. The greater the elevation, the more material will be available in thewedge, leading to greater penetration [5].

During the coating process, many compounds, because of their surface ten-sion properties, rise up the back of a knife, accumulate, and drop on the coatedsurface in an unsightly fashion, called spitting. In the shoe knife, due to thedesign of the toe, this is completely prevented. For PVC pastes and breathablePU coatings, shoe knife is the preferred type [6].

Proper positioning of the knife and the roll are other important considerationsfor proper coating. The rolls should be true to knife surface, without eccentricity.It is also vital that the knife be aligned horizontal to the axis of the roll, otherwise,wedge-shaped coating will result. The angle of the knife over the roll affectspenetration. The greater the angle at which the knife meets the moving fabric,the greater the penetration. If the position of the blade is at a point behind thecrown of the roll, the blade will be directly pressing the fabric, and a situationsimilar to floating knife is created [5].

Instead of a single knife fitted over the roll, modern machines have a knifesupporting beam fitted with two or three different types of knives mounted 180◦

or 120◦ apart. A twin knife arrangement is shown in Figure 3.7. This facilitateseasy changeover of the blades.


As has been described earlier, the coating compound is pumped or ladledover the substrate in front of the blade in knife coating, forming a rolling bankthat acts as a reservoir for the coating material. The viscosity and the amountof rolling bank also contribute to the penetration and coating weight of the endproduct. The rolling bank (Figure 3.8) exerts a pressure on the web, as such,if the height of the roll bank is greater at the center, heavier coating will beproduced at the center. Similarly, if the height is more at the sides, the coatingwill be more in the sides. In case of a pump pouring fluid over the web acrossthe width, its traverse also results in variation of coating in an “S” pattern.

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Figure 3.7 Twin knife arrangement.

Harrera has described Mascoe’s patented trough system (Figure 3.9) forbetter control of the conditions prevailing in the rolling bank [7]. The coatingcompound is fed into a trough in front of the blade. The gap between the troughand the blade is adjustable, but it is fixed during the coating process.

In this device, because the opening of the trough is constant, the exposureto fabric is controllable and is much shorter than the rolling bank. It is claimedthat greater accuracy and repeatability can be obtained by this system, however,cleanup of the trough is a problem.

Fabric tension plays an important role in the final add on of the coated productand is thus dependent on the stretchability of the fabric. A higher tension inwarp direction opens up the weave, exposing more surface, and thus, the coatingweight add on is heavier when the fabric is excessively stretched. This is trueif the weave pattern is regular. In case of an irregular pattern, application ofuniaxial tension results in uneven tension in the filling yarn, causing unevencoatings. Mascoe has developed a tensioner that maintains a uniform forceacross the substrate width, which is not altered by the changing speed of theweb [7].

Figure 3.8 Rolling bank: (1) web, (2) rolling bank, and (3) knife. (Adapted with permission fromA. Harrera. Journal of Coated Fabrics, Vol. 20, April 1991. c©Technomic Publishing Co., Inc. [7].)

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Figure 3.9 Mascoe’s trough: (1) web, (2) knife, (3) trough, (4) coating compound, (5) adjustablegap, and (6) feeder. (Adapted with permission from A. Harrera. Journal of Coated Fabrics, Vol. 20,April 1991. c©Technomic Publishing Co., Inc. [7].)



In this method, compound is applied on the web by a single-roll applicator.The coating is postmetered by a wire wound rod, known as the Mayer rod, thatremoves excess coating (Figure 3.10).

The Mayer rod is a small, round stainless rod, wound tightly with a finewire also made of stainless steel. The grooves between the wire determinethe precise amount of coating that will pass through. The coating thickness isdirectly proportional to the diameter of the wire. The most common core roddiameter varies from 4–6 mm, although sizes up to 25 mm are used. To preventdeflection of the Mayer rod due to web pressure, it is mounted on a rod holder.The simplest rod holder is a rectangular steel bar with a “V” groove machinedto it. The rod is placed on the groove, and the holder is mounted between theside frames of the coating machine. During coating, the rod is slowly rotated

Figure 3.10 Mayer rod coater: (1) applicator roll, (2) Mayer rod with holder, and (3) feed pan.(Adapted with permission from Encyclopedia of Polymer Science & Engineering, Vol. 3, 2nd Ed.1985, c©John Wiley & Sons.)

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Figure 3.11 Direct roll coater: (1) applicator roll, (2) doctor blade, and (3) backup roll.

in the opposite direction of the web. The rotation removes the coating materialbetween the wires, keeping the wire surface wet and clean. The rotation alsoincreases the life of the rod due to reduced wear.

The thin lines formed during coating smooth out due to surface tension. Theuniformity of the coating is maintained if the viscosity of the compound, speed,and tension of the web are properly controlled. This method is used for lowsolid, low viscosity (50–500 cps), thin coatings (2–3 g/m2). It is suitable forsilicone release papers and as a precoater [8].


In direct roll (or squeeze roll) coating, a premetered quantity of the coatingis applied on the fabric by controlling the quantity on the applicator roll bythe doctor knife (see Figure 3.11.) The fabric moves in the same directionas the applicator roll. This method is also restricted to low viscosity compoundsand is suitable for coating the undersurface of the fabric. The coating thicknessdepends on nip pressure, coating formulation, and absorbency of the web [2].


A typical arrangement of kiss coating is shown in Figure 3.12.The pickup roll picks up coating material from the pan and is premetered by

the applicator roll. The coating is applied on the web as it kisses the applicatorroll. The pickup roll may be rubber covered, and the applicator roll may bemade of steel. The metering is done by nip pressure, and consequently, the

Figure 3.12 Kiss coater: (1) pickup roll and (2) applicator roll. (Adapted with permission fromEncyclopedia of Polymer Science & Engineering, Vol. 3, 2nd Ed. 1985, c©John Wiley & Sons.)

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Figure 3.13 Gravure coater: (1) gravure roll, (2) backup roll, (3) doctor blade, and (4) smootheningrolls. (Adapted with permission from Encyclopedia of Polymer Science & Engineering, Vol. 3,2nd Ed. 1985, c©John Wiley & Sons.)

amount of material coated on the web is dependent on nip pressure, speed ofthe operation, roll hardness, and its finish. The coating weight and splitting ofthe film as it leaves the roll are also dependent on web tension.


Engraved rollers are utilized in gravure coatings to meter a precise amountof coating on the substrate. The coating weight is usually controlled by theetched pattern and its fineness on the gravure roll. There are a few standardpatterns like the pyramid, quadrangular, and helical. For lighter coating weight,a pyramid pattern is used. In a direct two-roll gravure coater (Figure 3.13), thecoating material is picked up by the gravure roll and then transferred to the webas it passes between the nip of the gravure and the backup roll. The pattern maybe self-leveling or the coated web may be passed between smoothening rolls.

In offset or indirect gravure coater, a steel backup roll is added above thedirect gravure arrangement. The coating compound is first transferred onto anoffset roll and then onto the substrate (Figure 3.14).

Figure 3.14 Offset gravure coater: (1) gravure roll, (2) rubber-covered offset roll, (3) steel backuproll, and (4) doctor blade. (Adapted with permission from Encyclopedia of Polymer Science &Engineering, Vol. 3, 2nd Ed. 1985, c©John Wiley & Sons.)

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Figure 3.15 A gravure coating plant. Courtesy Polytype, U.S.A.

The speed and direction of the gravure and offset rollers can be varied inde-pendently. The arrangement is suitable for an extremely light coating (as lowas 0.02 g/m2) and minimizes the coating pattern. This offset process can handlea higher viscosity material (∼10, 000 cps.) than the direct process. By heatingthe feed pan, the process can coat hot melt compounds. Gravure coating is usedfor applying laminating adhesives or a topcoat on a treated fabric. A gravurecoating plant is shown in Figure 3.15.


Reverse roll coating is one of the most versatile and important coating meth-ods. It can be used for a wide range of viscosities and coating weights. Theaccuracy of the coating is very high. Reverse roll coaters apply a premeteredcoating of uniform thickness, regardless of the variations in substrate thick-ness, and are therefore known as contour coaters. The coating is also indepen-dent of substrate tension. There are two basic forms of reverse roll coaters:three-roll nip and pan fed. Figure 3.16 shows the arrangement of a nip-fedcoater.

The applicator and the metering rolls are precision-ground chilled cast ironor stainless steel rolls, finished to a high degree of precision. The two rollsare set at an angle, and the coating material is kept in a reservoir, at the nip,bound by the applicator roll and coating dams on each side. The gap betweenthe applicator and metering roll can be precisely controlled. The backup roll isto bring the moving web in contact with the applicator or transfer roll. A film

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Figure 3.16 Nip-fed reverse roll coater: (1) applicator roll, (2) metering roll, (3) backup rubberroll, (4) web, (5) coating pan, (6) doctor blades, and (7) drip pan. (Adapted with permission fromEncyclopedia of Polymer Science & Engineering, Vol. 3, 2nd Ed. 1985, c©John Wiley & Sons.)

of the coating compound is metered between the applicator and the meteringroll. The applicator roll then carries the coating material to the coating nipwhere the compound is transferred to the web moving in the opposite direction.The opposite direction of the applicator roll and the web creates a high level ofshearing action. The criteria of a reverse roll coater are the opposite direction(a) of the applicator and the metering roll and (b) of the applicator roll and theweb. A scraper or doctor blade cleans the metering roll to prevent dropping ofmaterial on the web. The coating material remaining on the applicator roll aftercontact with the web is also scraped off, collected in a pan, and recycled. Thishelps to clean the roll of dirt and dried coating material, which would causeinaccuracy in coating.

The thickness of the coating is controlled by the gap between the applicatorand the metering roll, the rotational speed of the applicator roll, and the amountof material transferred on the web, which in turn is dependent on the webpressure on the applicator roll adjusted by the backup roll. Thus, a reverse rollcoater has greater flexibility in adjusting the coating thickness compared to theknife-on-roll, where coating is controlled only by gap of knife and the roll. Indirect roll coating, where the web and applicator roll move in the same direction,nonuniform coating occurs with the formation of ribbing due to the film splitphenomenon, while in reverse roll, the coating is smooth [6].

One challenge when using the nip-fed coater is to prevent leaks from the coat-ing reservoir, particularly with low viscosity compounds. The pan-fed coateroperates using the same principle as the nip-fed coater, but it is more suited forlow viscosity materials (Figure 3.17).

Greer [9] has recently reviewed studies on the fluid mechanics of reverse rollcoating. The reasons for nonuniformity of coating have been discussed. The twocommon defects are ribbing and cascading, i.e., formation of a wavy pattern. To

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Figure 3.17 Pan-fed reverse roll coater: (1) metering roll, (2) applicator roll, (3) web, (4) doctorblade, and (5) backup roll. (Adapted with permission from Encyclopedia of Polymer Science &Engineering, Vol. 3, 2nd Ed. 1985, c©John Wiley & Sons.)

explain the reasons for these defects, a dynamic wetting line has been defined.This is the point from which the coating pulls away from the metering roll as itleaves the metering nip and moves along with the applicator roll. If the wettingline is at the center of the nip, the coating is smooth. If the wetting line is atthe outlet side, ribbing occurs; if it is on the inlet side, cascading occurs. Animportant criterion governing the position of the wetting line is the ratio ofmetering to applicator roll speeds. In addition, surface tension and viscosity ofthe coating compound also play important roles.


This is also known as impregnation or saturation. The substrate web is im-mersed in a tank of the coating material for a certain period of time, known asthe dwell time. The excess material is then squeezed out by passing through niprolls or a set of flexible doctor blades precalibrated to give a fixed net pickupof the resin. There are various arrangements for the dip process. A simple ar-rangement is shown in Figure 3.18. Sometimes, a prewet station precedes thedipping to remove air from the interstices and promote penetration. The fac-tors that are to be considered in designing an impregnated fabric are the solidcontent of the impregnant and the absorption capacity of the fabric. In dip coat-ing, the pickup is quite low, and penetration occurs into the interstices of thefabrics as well as in the yarns. Moreover, because the fabric is not stressed,

Figure 3.18 Dip coating: (1) squeeze rolls, (2) web, and (3) dipping tank. (Adapted with permissionfrom Encyclopedia of Polymer Science & Engineering, Vol. 3, 2nd Ed. 1985, c©John Wiley & Sons.)

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no damage or distortion to the yarn occurs. The process is mainly used forfinishing processes like flame retardant treatments and application of adhesiveprimer.


In principle, transfer coating consists of applying polymeric coating on thesurface of a support, usually paper, laminating the textile substrate to be coatedto the polymeric layer, and removing the paper, to yield a transferred polymericlayer on the textile.

The process of applying coating material directly on the textile is known asdirect coating. The direct coating process has certain limitations. They are asfollows:

� It is applicable to closely woven, dimensionally stable fabrics that canwithstand machine tension, and it is not suitable for excessivelystretchable knitted fabrics.

� Penetration occurs in the weave of the fabric, increasing adhesion andlowering tear strength and elongation, resulting in a stiff fabric.

Transfer coating overcomes these limitations. Because no tension is appliedduring coating, the most delicate and stretchable fabrics can be coated by thisprocess. Fabric penetration and stiffening is significantly low. Moreover, withproper processing, the appearance of the textile substrate can be altered to givea much better aesthetic appeal, like artificial leather for fashion footwear. Aschematic diagram of the process is given in Figure 3.19.The steps involved are as follows [1]:

� A layer of coating is applied on a release paper in the first coating headand is then passed through the first oven, where it is dried and cured.This forms the top surface of the coated fabric. The release paper is

Figure 3.19 Layout of transfer coating process: (1) release paper, (2) first coating head, (3) firstoven, (4) second coating head, (5) textile substrate, (6) laminating nip rolls, (7) second drying oven,(8) coated fabric takeoff roll, and (9) release paper wind roll. (Adapted with permission from G. R.Lomax, Textiles, no. 2. 1992. c©Shirley Institute U.K. [1].)

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usually embossed. The pattern of the paper is thus transferred on thecoating. This is known as the top coat.

� In the second coating head, an adhesive layer known as the tie coat isapplied on the dry top coat, previously laid on the release paper. Therelease paper thus has two layers, the dry top coat and the tacky adhesivetie coat.

� The textile substrate is then adhered to the release paper containing thetop and the tie coats, while the tie coat is still tacky. The lamination isdone by a set of nip rolls. The composite layer is then passed through thesecond oven to dry and cure the tie coat.

� The release paper is finally stripped, leaving the coated textile.

The release papers are calendered to uniform thickness and are coated witha thin layer of silicone. The property of release paper should be such that it isable to grip the top coat during the processing and able to release the fabricwithout damaging the top coat. Various grades are available. Normally, papercan be reused about eight to ten times.

The coating head in a transfer coating unit is typically knife over rubber-backing roll. The rubber roll has the advantage in that it does not damage therelease paper. The laminator rolls are steel rolls. The setting of the coatingknives and gap between the laminating rolls can be set and maintained fullyautomatically.

Transfer coating is used for PVC pastes and for polyurethane coating. Al-though the basic transfer process involves a two-coat operation, the top and thetie coat, a three-coat process is becoming quite popular. The first two headsapply the top coat in two thin layers. This permits faster line speeds due togreater efficiency of solvent removal from thinner films, and it prevents pin-holing, where waterproofness is important. The third coating head applies thetie coat. In polyurethane transfer coating, this affords an option of using twodifferent types of PU for the two layers of top coats for special properties, asrequired for artificial leather [10].


This method is common for coating and printing textiles. The coating head isa screen that is a seamless nickel cylinder with perforations. This screen rests onthe web. A squeegee is mounted in the screen, serving as supply and distributionpipe of the coating paste. The squeegee blade, which is mounted to this pipe,pushes the paste out through the perforations of the screen. A whisper bladesmooths the applied coating. A backup roll is provided for counterpressure(Figure 3.20). After coating, the coated material is sent to an oven for fusionof the polymers. The amount of coating applied is determined mainly by the

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Figure 3.20 Rotary screen coating: (1) web, (2) squeegee, (3) screen, (4) whisper blade, and (5)backup roll.

mesh number of the screen, the squeegee pressure, i.e., the angle between theblade and the screen, and the viscosity of the paste.

Depending on the mesh size and design of the screen, continuous coating,coating of complex pattern, and dot coating can be done. In continuous coating,the coating can be up to 200 g/m2, by proper choice of the screen. Dot coating isuseful for making fusible interlinings for woven and nonwoven fabrics. In thisprocess, the screen, the web, and the counterpressure roller all have same speed.The coating is, therefore, done without tension and friction. Consequently,delicate and stretchable fabrics can be coated without difficulty. The coating isaccurate, and the penetration can be controlled [8].

A relatively new development by Stork (Stork-Brabent, Holland) isthe screen-to-screen technology (STS). Basically, the process consists of twoscreen-coating heads, back to back, each with its own coating feed system,squeegee roll, and whisper blade to smooth out the applied compound. Thesubstrate travels between the screens either in a horizontal or a vertical posi-tion (depending on the model), and the compound is gelled (or cured) with IRheaters. With STS technology, it is possible to coat (different colors) or printboth sides of a substrate in one pass.


Calendering is a versatile and precise method of coating and laminatingpolymeric material onto a fabric. The equipment consists of a set of heatedrolls also known as bowls. Fluxed, precompounded stock is fed between theroll nips, which comes out as a sheet as it passes through consecutive roll nips.The sheet so produced is press laminated to the fabric with another pair of matingrolls, which may be in the same calender machine. A variety of thermoplasticscan be processed on the calender, however, it is extensively used for coatingrubbers and vinyls.

For continuous operation of a calender, different equipment is arranged inline and functions in tandem to produce the coated fabric. This is known as

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Figure 3.21 Precalender train.

the calender train. The train has three distinct sections, viz, the precalenderingsection, the calendering itself, and the postcalendering section.


The task of the precalender section is to deliver fused polymer stock to thecalender in a thoroughly compounded, homogenized, degassed condition thatis free from impurities. The compounding can be done either in a batch pro-cess, using an internal mixer such as the Banbury mixer, or continuously, inan extruder. The compounded material is then fed to a two-roll mill to im-part uniformity of temperature to the stock. Frequently, the material is nextpassed through a strainer. The strainer is a heated short-barreled extruder witha screen orifice. It masticates the compound further and prevents impurities,particularly metal particles, from entering the calender. The compounded ma-terial is then fed into the calender by a conveyor. The feed is a thick andnarrow strip of material. Metal detectors are provided in the feed line to pre-vent any metal particle from entering the calender and damaging the rolls. Ifthe material is conveyed a distance of more than 2 m, the feed is heated byIR heaters. The stages of this section are shown in the block diagram given inFigure 3.21.


The equipment consists of a stack of rolls mounted on bearing blocks, sup-ported by a side frame. It is equipped with roll drive, nip adjusting gear, andheating arrangement. The rolls are made of chilled cast iron. The number ofrolls and their arrangements are varied. Three- and four-roll calenders are themost popular.

Thinner coating can be achieved by increasing the number of rolls, but theyincrease the complexity of the equipment, the cost, and the space required. Thesizes of the rolls vary from 45–120 cm diameter and 90–300 cm width [6,11,12].

Some common configurations of the rolls are given in Figure 3.22.Vertical arrangements of the rolls in the stack were used in early machines.

They suffer from the problem of adjusting the nips independently and of feedingthe calender [13]. The feeding is easier if the top roll is offset, in an inverted“L”-type configuration, for instance, the feed bank is horizontal.

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Figure 3.22 Configurations of rolls.

The selection of the configuration of rolls is dependent on its end use. Themost suitable configuration for plasticized vinyl compounds is the inverted “L”,the most suitable for rubbers is the three-roll inclined, and the most suitable fortwo-sided coating is the “Z” type [14].

The rolls are individually driven, which provides wide flexibility in thevariation of the roll speeds and the corresponding friction ratio. For propercontrol of process temperature, the rolls are heated. The rolls are providedwith either a hollow chamber or they have peripheral holes located closeto the roll surface. The heating is done by circulation of hot water or spe-cial heat exchange liquid. Temperature control in the peripheral holes is farsuperior due to better heat transfer. The two factors, viz., the friction ratioand temperature, enable the calender to process a wide range of composi-tions differing in rheological properties. End dams are fitted in the rolls toconstrain the compound in an adjustable span for coating fabrics of differentwidths [6,12].

The compound is fed into the feed nip of the calender. In the case of aninverted “L” type, it is the nip between the top two rolls. The rolls rotate inopposite directions at the nip and at different speeds. The material is pushedforward by the friction of the rolls and adheres to the faster roll. The materialthen passes through the successive rolls where it is resurfaced and metered andcomes out of the calender in the form of a sheet. Calendering can, therefore, beregarded as sheet extrusion. During operation, rolling banks are formed at eachnip. It is thick and narrow at the feed nip but becomes thin and wide in successivenips. The passage of the material from the feed to outlet is known as the sheetpath and is controlled by the adherence of the material to a particular roll. Thematerial adheres to the faster roll and the one having a higher temperature.

Separating forces on the rolls are produced when the viscous material ispassed through the nip. These forces are highest at the center of the roll,

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therefore, the sheet is thicker in the middle. The separating force is dependenton several factors: the viscosity of the material, the gap between the rolls, thespeed of the rolls, the size of the rolling bank, etc. In older machines, this wascorrected by contouring the rolls in such a way that the center of the rolls had aslightly larger diameter, known as roll crowning. However, this is only suitablefor specific compounds and operating conditions [15]. The methods adoptedat present are roll bending and roll crossing. In roll bending, a hydraulic loadis applied to the roll journal ends. This force exerts leverage on the rolls thatmakes them slightly concave or convex depending on the direction of the ap-plied load. In roll crossing, an angular shift is given to one or both rolls at thenip. Although the rolls remain in the horizontal plane, their axes are no longerparallel but form a slight angle. This increases the end clearance between therolls, resulting in thickening at the edges of the sheet produced. For proper op-eration, the material should be fed at a steady rate at uniform temperature. Anyvariation results in roll deflection and consequent variation in the thickness ofthe product.


A calender is used for coating polymer directly onto the fabric or for makingunsupported film that may be subsequently laminated to a fabric. There arevarious ways of coating and lamination which are discussed below [6,11].

a. Nip coating: here, the coating is done at the bottom nip of the calender.The mechanical setup is simpler. The fabric pulls the sheet off the calender.The method is suitable for heavy impregnation. The extent of penetration isdependent on the gap at the nip and the friction ratio (Figure 3.23).

b. Lamination against calender roll: in this setup, the fabric is laminated againstthe last calender roll by means of a rubber backup laminating roll that ishydraulically operated—known as a squeeze roll. The conditions of pene-tration and takeoff of the sheet from the calender are similar to those in niplamination (Figure 3.24).

Figure 3.23 Nip coating: (1) fabric, (2) rubber bank, and (3) coated fabric. (Adapted with permis-sion from PVC Plastics by W. V. Titow. c©Kluwer Academic Publishers, Netherlands, 1990.)

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Figure 3.24 Lamination against calender roll: (1) squeeze roll, (2) fabric, and (3) coated fabric.(Adapted with permission from PVC Plastics by W. V. Titow. c©Kluwer Academic Publishers,Netherlands, 1990.)

c. In-line lamination: the sheet produced from the calender is laminated to thefabric outside the calender by laminating rolls. This arrangement is con-venient for heat-sensitive substrates. The sheet coming out of the calendermay cool and have to be heated prior to lamination for proper bonding to thefabric (Figure 3.25).

d. Another method of laminating multiple sheets of polymer and textile is givenin Figure 3.26. Here, two or more sheets of polymer and textile are laminatedby pressing them between a steel belt and a hot roll. The heat and pressurelaminate the webs. The steel belt (1) is pressed against a hot drum (2) bymeans of a tension roll (3) and guide rolls (4 and 5). The fabric and the sheetsare heated by IR heaters prior to being fed between the gap of the steel beltand the hot roll. The configuration is similar to the rotocure system used incontinuous vulcanization.

e. Coating of elastomers: as mentioned earlier, the rolls rotate in the oppositedirection at the nip with different speeds. The higher the friction ratio, thegreater the penetration. Thus, if the rolls run at even or near even speeds,the penetration is low, and the coating thickness is high. For rubberizedfabrics requiring thick coatings with high degrees of penetration for betteradhesion, a friction coating is applied first, followed by top or skim coating.The frictioning is done at a higher temperature and at a friction ratio of

Figure 3.25 In-line lamination: (1) sheet, (2) fabric, and (3) hydraulically operated laminatingroll. (Adapted with permission from PVC Plastics by W. V. Titow. c©Kluwer Academic Publishers,Netherlands, 1990.)

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Figure 3.26 Lamination against steel belt: (1) steel belt, (2) hot roll, (3) tension roll, (4, 5)guide rolls, (6, 7, 8) IR heaters, (9, 11) polymer sheets, and (10) textile sheet. (Adapted withpermission from D. Zickler. Journal of Coated Fabrics, Vol. 8, Oct. 1978. c©Technomic PublishingCo., Inc. [16].)

1:1.5 to 1:2; for skim coating, the friction ratio is 1:1.1 to 1:2. The operatingtemperature of the calender depends on the polymer, however, it is generallybetween 60◦ to 150◦C. For rubber coating, the temperature required is lowerto prevent scorching. A three-roll inclined calender is suitable for rubbercoating (Figure 3.27) [14].

Simultaneous coating on both sides in a “Z”- type inclined calender is shownin Figure 3.28 [14].


A block diagram of the post-calender section is given in Figure 3.29.Embossing consists of a pair of rolls, one of them is a metal engraved roll to

impart the pattern, and the other is a rubber-covered roll. The coated fabric fromthe calender is passed through the nip of the rolls for embossing. The diameterof the roll is determined by the size of the repeat pattern. The rubber roll mayhave the same diameter or be larger than the metal roll.

The thickness of the sheet is measured by β-ray gauge. The feed from thegauge is used to automatically control roll bending or crossing to correct thethickness variation.

The finished product is then passed over cooling cans to lower the temperatureand reduce tack. The train consists of a set of cans that is cooled by circulating

Figure 3.27 Three-roll inclined calender for rubberized fabric: (1) fabric, (2) rubber bank,(3) laminating roll, and (4) coated fabric. (Adapted with permission from J. I. Nutter. Journalof Coated Fabrics, Vol. 20, April 1991. c©Technomic Publishing Co., Inc. [14].)

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Figure 3.28 Coating on both sides in a “Z” calender: (1) rubber bank, (2) laminating roll, (3)fabric, and (4) double-sided coated fabric. (Adapted with permission from J. I. Nutter. Journal ofCoated Fabrics, Vol. 20, April 1991. c©Technomic Publishing Co., Inc. [14].)

cold water through its inner shell. The number of cans depends on the sheetthickness and the speed of operation [6,11].

The fabric is then wound up in rolls.


One of the major problems of processing vinyl compounds is plate-out. Itis the transfer of a sticky deposit that sometimes appears on the rolls of thecalender. The calender rolls, embossing rolls, and cooling rolls require frequentcleaning in order to prevent loss of output and surface defects. Plate-out occursfrom hot PVC stocks due to an incompatibility of some the constituents of thecompound, particularly certain lubricants and stabilizers.

Other surface defects, like discoloration, surface roughness, crowfeet marks,bank marks, etc., are caused by diverse factors, such as excessive heat on thecompound, nonuniformity of the stock temperature, improper dispersion ofparticulate additives, and undergelation of PVC [11,15].


The calendering process imparts an even coating within a range of about0.1–1.5 mm. The upper limit is restricted by the formation of blisters on thecoated fabric due to entrapment of air in the rolling bank. For lower thickness,the load requirement on the calender is heavy, moreover, at lower thickness, voidformation takes place. An extruder can produce thicker gauges suitably; but, in

Figure 3.29 Post-calender section.

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thinner gauges, there is abrupt variation in thickness [6,13]. The advantage ofthe calender over an extruder is its (a) high production rate, (b) better productthickness control, and (c) suitability for continuous operation. Compared tofluid coating by spreading, calender coating is a much cleaner process, doesnot require removal solvents, and uses raw materials that are lower in cost,particularly for vinyl compositions. The cost of calender equipment is, however,higher than either extrusion or spreading [6,14].


The Zimmer coater (Zimmer Plastics GmbH, Germany) and Bema coater(A. Manrer, Switzerland) are calender-like machines that have been specifi-cally designed for coating fabrics. Thermoplastic polymers in the form ofgranules, dry powder, or plastic stock are the feedstock for these machines.These machines are less expensive and require lesser manpower and space thana calender.

The Zimmer coater (Figure 3.30) consists of two melt rolls (1) and(2). Therolls are made of diamond-polished, deep-hardened high-grade steel. The gapbetween the two rolls is adjusted hydraulically. The material is fed at the nip ofthe rolls, the temperature of which is about 200◦C. The coating material meltsand adheres to roll (2) which runs at a higher speed and is maintained at a highertemperature. After heating by passage through one or more preheater rolls andIR heaters, the textile substrate is fed between the nip of roll (2) and the backuproll (3). After coating, the hot laminate is either smoothened or embossed by

Figure 3.30 Zimmer coater: (1, 2) melt rolls, (3) backup roll, (4) embossing roll, (5) coolingroll, (6) substrate preheat roll, (7) fabric roll, and (8) IR heaters. (Adapted with permission fromD. Zickler. Journal of Coated Fabrics, Vol. 8, Oct. 1978. c©Technomic Publishing Co., Inc. [14].)

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an embossing roll (4). The coated material is then cooled by cooling drums andwound [16].



In this process, an extruder converts solid thermoplastic polymers into amelt at the appropriate temperature required for coating. This melt is ex-truded through a flat die vertically downward into a nip of the coating rolls(Figure 3.31).

The two rolls at the nip are a chromium-plated chill roll and a soft, high-temperature-resistant elastomer-coated backup roll. The chill roll is watercooled. The heat transfer should be adequate to cool the coated fabric so that itcan be taken out of the roll smoothly. Means are provided to adjust the positionof the die and the nip in three directions. The chill roll may be polished, mattfinished, or embossed. Lamination can be accomplished by introducing a sec-ond web over the chill roll. The molten resin acts as an adhesive. Extrusioncoating is especially suitable for coating polyolefins on different substrates.Because polyolefins can be brought down to low viscosity without risk of de-composition, very high coating rates are achieved, and as such, the process ishighly economical. For other polymeric coatings like PVC, PU, and rubber,this process does not yield uniform coating across the width, particularly atthickness below 0.5 mm.

In this method, the coating width can be adjusted by reducing the apertureof the die by insertion of shims. Thus, it is possible to coat different widthsfor a given die, however, the coating width cannot be changed while coating.Moreover, the process does not permit easy changeover of material. This re-stricts its use for coating industrial fabrics [16]. A view of an extrusion coatingplant is shown in Figure 3.32.

Figure 3.31 Extrusion coating: (1) extruder, (2) die, (3) chill roll, (4) backup roll, and (5) pressureroll. (Adapted with permission from Encyclopedia of Polymer Science & Engineering, Vol. 3,2nd Ed. 1985, c©John Wiley & Sons.)

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Figure 3.32 Extrusion coating plant. Courtesy Egan Davis Standard Corp., U.S.A.


There are two processes in this category, scatter coating and dot coating[17,18]. These processes are used for coating fusible polymer powder. Theyare polyethylene, polyamide, polyester, and EVA. The products are used forfusible interlinings, carpet backcoating, especially in the automotive industryfor contoured car carpets, and for lamination. The process lends itself to thelamination of two different types of webs, e.g., textiles to foam. Laminatesproduced by this process retain their flexibility and porosity. Scatter coating isalso used for fiber bonding of nonwovens.

In the scatter coating process, polymer powder of 20–200 µm size is spreaduniformly onto a moving textile substrate. The web is then passed through afusion oven and calendered. The method of scattering the powder may be avibrating screen or a hopper with a rotating brush arrangement, the latter beingmore accurate. The coating weight is dependent on feed rate and web speed(Figure 3.33).

Figure 3.33 Scatter coating: (1) hopper, (2) rotating brush, (3) fabric let off, (4) IR heater, and(5) two-roll calender. (Adapted with permission from Encyclopedia of Chemical Technology,Vol. 6, 3rd Ed. 1979, c©John Wiley & Sons.)

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Figure 3.34 Powder dot coating: (1) fabric, (2) oil-heated drum ∼200◦C, (3) engraved roll,(4) powder feed, (5) cleaning brush, (6) IR heaters, and (7) chilled drum. (Adapted with permissionfrom C. Rossito. Journal of Coated Fabrics, Vol. 16, Jan. 1987. c©Technomic Publishing Co.,Inc. [18].)

In the powder dot coating process, a heated web having a surface temperatureslightly less than the melting point of the polymer is brought in contact withan engraved roller embedded with dry powder. The web is thus coated with atacky polymer powder in a pattern dependent on the engraving. The engravedroller is kept cool to prevent the polymer from sticking to the roll. A schematicdiagram is shown in Figure 3.34 [18].

A new method for printing a hot-melt on a web has been recently described byWelter [19]. In this process, hot molten polymer contained in a trough attachedto an engraved roller is picked up by the latter and is pressed into a running web,with the pressure coming from a backup roll. Lamination can also be achievedwith another web in the same machine. Various patterns can be printed onthe web, including the conventional dot and computer dot printing (Figure 3.35).

Hot-melt coating offers certain advantages over fluid coating. These are asfollows [20]:

(1) It does not pollute the environment as no volatiles are emitted.

(2) The rate of production is higher, as it is not dependent on rate of dry-ing/curing.

(3) The plant space requirement is smaller as drying ovens are not required.

(4) The energy consumption is low.

(5) It has better storage stability than fluids.

Figure 3.35 Engraved roller melt printing: (1) fabric, (2) backup roll, (3) engraved roll, (4) polymermelt, (5) trough, (6) second web, (7) preheater, and (8) calender. (Adapted with permission fromC. Welter. Journal of Coated Fabrics, Vol. 24, Jan. 1995. c©Technomic Publishing Co., Inc. [19].)

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1. G. R. Lomax, Textiles, no. 2, 1992, p. 18.2. Spread coating processes, R. A. Park, in Plastisols and Organosols, H. A. Sarvet-

nick, Ed., Van Nostrand Reinhold, New York, 1972, pp. 143–181.3. W. R. Hoffman, Journal of Coated Fabrics, vol. 23, Oct., 1993, pp. 124–130.4. Coated fabrics, B. Dutta, in Rubber Products Manufacturing Technology, A. K.

Bhowmik, M. M. Hall and H. A. Stephens, Eds., Marcel Dekker, New York, 1994.5. F. A. Woodruff, Journal of Coated Fabrics, vol. 21, April, 1992, pp. 240–259.6. Encyclopedia of PVC, L. I. Nass, Ed., vol. 3, Marcel Dekker, New York, 1977.7. A. Harrera, Journal of Coated Fabrics, vol. 20, April, 1991, pp. 289–301.8. Wire wound rod coating, D. M. MacLeod, in Encyclopedia of Coating Technology,

D. Satas, Ed., Marcel Dekker, Inc. New York, 1991.9. R. Greer, Journal of Coated Fabrics, vol. 24, April, 1995, pp. 287–297.

10. V. E. Keeley, Journal of Coated Fabrics, vol. 20, Jan., 1991, pp. 176–187.11. PVC Plastics, W. V. Titow, Elsevier Applied Science, London, New York, 1990.12. G. W. Eighmy, Journal of Coated Fabrics, vol. 12, April, 1983, pp. 224–236.13. Rubber Technology and Manufacture, C. M. Blow, Ed., Butterworths, London, 1971.14. J. I. Nutter, Journal of Coated Fabrics, vol. 20, April, 1991, pp. 249–265.15. Poly Vinyl Chloride, H. A. Sarvetnick, Van Nostrand Reinhold, New York, 1969.16. D. Zickler, Journal of Coated Fabrics, vol. 8, Oct., 1978 pp. 121–143.17. M. H. Luchsinger, International Textile Bulletin, 1/89, 1989, pp. 5–16.18. C. Rossitto, Journal of Coated Fabrics, vol. 16, Jan., 1987, pp. 190–198.19. C. Welter, Journal of Coated Fabrics, vol. 24, Jan., 1995, pp. 191–202.20. J. Halbmaier, Journal of Coated Fabrics, vol. 21, April, 1992, pp. 201–209.

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Physical Properties of Coated Fabrics


COATED textiles are flexible composites, consisting of a textile substrate anda polymeric coating. The coating may be on one side or on both sides

with the same or a different polymeric coating per side. A typical construction,coated on both sides, is depicted in Figure 4.1.

The physical properties of a coated fabric depend on the properties of thesubstrate, the coating formulation, the coating technique, and the processingconditions during coating. The factors responsible for different properties of acoated fabric are given in Table 4.1 [1] .

Due to the application of longitudinal tension during the coating process, theposition of the yarns in the textile substrate is considerably altered in both thewarp and weft directions. The warp yarns are aligned more parallel, whereasin the weft there is an increase in the crimp. The minimum coating thickness isthus on the top of the filling yarns.


The strength of a fabric depends on type of fiber, fineness, twist, and tenacityof yarns and also on the weave and yarn density (set). Theoretically, the tensilestrength of a fabric should be the sum of the tensile strength of all the yarnsadded together. However, there is always a loss of strength due to weaving, andas a result, the theoretical strength is never achieved. The conversion penaltydue to weave has been calculated and reported in a recent study on plain weavepolyester fabric of varying yarn densities [2]. It has been found that the pro-cessing penalty in the warp direction is about 10% and in the weft direction isabout 15%. The processing penalty increases with yarn density of the fabric.

The reason for this conversion loss is due to the thread strain during theweaving process, i.e., shedding, warp formation, weft insertion, etc., and due to

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Figure 4.1 A double-sided coated fabric.

the transversal strain at the intersection points. The higher weaving penalty inthe weft is due to greater waviness of the weft yarn bending around the stretchedwarp yarns.

Earlier workers observed that coating increased tensile strength [3–5], how-ever, the reasons for the same were not properly explained. This aspect was thor-oughly investigated by Eichert by measuring the strength of coated polyesterfabric of different yarn densities [2]. It was observed that the tensile strength ofthe coated fabric increased from loomstate fabric when calculated on the basis ofnominal thread count. Eichert argued that because the difference of break elon-gation of the yarn and the coating compound is so great, the coating compoundcannot contribute to the tensile strength in any way. On further investigation, it

TABLE 4.1. Factors Affecting the Properties of Coated Fabrics.∗

Coating Technique/Properties Substrate Construction Processing Conditions Recipe

1. Tensile strength � �

2. Extension at break � �

3. Dimensional stability � �

4. Burning behavior � �

5. Long-time properties � � �

6. Coating adhesion � � �

7. Tear strength � � �

8. Bending resistance � �

9. Cold resistance �

10. Heat resistance �

11. Chemical resistance � �

12. Sea water resistance �

13. Weather resistance � �

14. Abrasion resistance �

15. Welding properties �

∗ Adapted with permission from U. Eichert, Journal of Coated Fabrics, vol. 23, April 1994,c©Technomic Publishing Co., Inc. [1].

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Figure 4.2 Tensile strength of coated fabric conversion loss % vs. effective set (Diolen 174 S 1100dtex 1210 Z 60). (Adapted with permission from U. Eichert. Journal of Coated Fabrics, Vol. 24,July 1994. c©Technomic Publishing Co., Inc. [2].)

was found that there is a shrinkage of fabric in the weft direction due to tensionand heat of the coating compound during the coating process. This resultedin an increase in yarn density in the warp direction. A comparison of tensilestrength of coated fabric, with that of loomstate fabric, computed from effectivethread count showed conversion loss in all fabric densities studied in the warpdirection. The conversion loss was found to be more than the weaving penalty.As per Eichert, other reasons for conversion loss due to coating are transversalstrain and heating of the yarns in the coating process. The conversion loss isshown in Figure 4.2.


The extension of break of coated polyester fabric of different fabric densitieshas been studied by Eichert (Figure 4.3) [2]. In the uncoated fabric, the elon-gation only shows a slight increase with thread count both in warp and weftdirections. In coated fabric, however, the elongation in the warp direction ismuch less than that in the loomstate fabric and is similar to the yarn elongation.This is due to the fact that during the coating process, the fabric is subjected tolongitudinal tension, stretching the warp threads taut and parallel. Due to thestretching of the warp threads, the looping angle of the weft thread increases,and this phenomenon increases with thread count. This pronounced looping orcrimp transfer causes enhanced elongation in the weft direction. Lower elonga-tion of basket weave both in loomstate and coated fabric in weft is due to muchlower interlacing.

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Figure 4.3 Elongation of loomstate vs. coated fabric (Diolen 174 S 1100 dtex 1210 Z 60). (Adaptedwith permission from U. Eichert. Journal of Coated Fabrics, Vol. 24, July 1994. c©TechnomicPublishing Co., Inc. [2].)


The forces of adhesion between the coating compound and the textile sub-strate are a combination of mechanical and chemical bonding, particularly whenbonding agents are added. Mechanical adhesion is predominant in staple fiberyarn and in texturized yarns. Acceptable adhesion values can be achieved bet-ween coating and these yarns without the addition of adhesion promoters des-cribed earlier. In the case of smooth, high tenacity filament yarns, mechani-cal adhesion is much lower, and chemical adhesion predominates. Chemicaladhesion is obtained by the interaction of the adhesive system with the polargroup of the textile substrate and the polymeric coating composition. The ef-fect of mechanical bonding is seen from a study of adhesion in PVC-coatedpolyester fabric of varying yarn densities (Figure 4.4) [2]. In very loose con-structions, the adhesion is very high due to mechanical factors, because ofstrike-through of the coating composition through the interstices of the fabric.The adhesion decreases with fabric density and becomes more or less level ascoating penetration decreases. Thus, even though adhesion on the yarn is at alow level, high adhesion due to mechanical factors can be achieved in scrimfabric.

Chemical adhesion is mainly brought about by treatment of the textile orincorporation of bonding agents in the coating material. The mechanism of theaction of the bonding agents has already been described in Chapter 1. Certaincoating materials such as polyurethanes and chloroprene also contain reactivegroups that promote adhesion. The adhesive system for rubber has already been

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Figure 4.4 Effect of yarn density on adhesion (Diolen 174 S 1100 dtex 1210 Z 60). (Adaptedwith permission from U. Eichert. Journal of Coated Fabrics, Vol. 24, July 1994. c©TechnomicPublishing Co., Inc. [2].)

described. The adhesive systems used in trade for coating different polymericformulations can be classified as follows:

(1) One-component system—polyfunctional isocyanates, e.g., Desmodur R(Bayer), Vulcabond VP (ICI), etc., suitable for rubbers, PVC, and polyure-thanes

(2) Two-component system—containing polyols and diisocyanates, e.g., Des-modur N, Plastolein, etc., suitable for PVC and polyurethanes

(3) Three-component system—RFL systems, mainly for rubbers

In vinyl coating, the bonding system is added to the plastisol in a range ofabout 4–6%. These bonding agents may be one- or two-component systems.The incorporation of the additives increases the viscosity of the PVC paste.Moreover, the plastisol temperature affects the pot life of the bonding agent.

Vulcabond VP [6] is an important bonding agent, for vinyl and textile sub-strates made from synthetic fibers. Chemically, it is a trimer of toluene di-isocyanate dispersed in dibutyl phthalate. The reactive -NCO reacts with thehydroxyl and amido groups of polyester and nylon, respectively, and with theactive hydrogen atom of the polyvinyl chloride chain, thus promoting adhesion.Optimum dose level of the additive is ∼4%. Higher bonding agent enhancesadhesion but causes loss of tear strength of the coated fabric.

The factors affecting adhesion can be summarized as follows [7]

� type of fiber� fiber surface: moisture, finish, etc.� construction of fabric

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� polymer for coating and its recipe� bonding agents� method of coating and coating conditions

Haddad and Black [8,9] have studied the effect of type of yarn, yarn con-struction, and fabric structure on adhesion. The fabric surface to be tested waspressed against a standard vinyl film. The peel tests were performed as perASTM 751-79. Effect of weave pattern on adhesion is given in Figure 4.5.

It is seen that adhesion is the same on both sides of a 2 × 2 twill beinga balanced fabric. In 1 × 3 twill, however, adhesion is higher on the fillingside. This is because the picks were unsized and made of core-spun yarns.The effect of yarns on adhesion was studied by making a series of 1 × 3 twillconstruction with textured air-entangled polyester as warp and by using differenttypes of filling yarns, viz., multifilament, core-spun, and spun yarns. The highestadhesion was obtained in air-textured yarns, yarns with cotton as the sheath,and open-end spun yarns in the three categories, respectively (the results aregiven in Table 4.2).

In the case of textured polyester yarns, a relationship between shrinkage(dependent on crimp and bulk) and adhesion has also been found. Clearly,adhesion is dependent on weave and nature of yarn. An open-weave structureand higher yarn surface exposure promote adhesion due to increased mechanicalanchoring of the coating compound.

Figure 4.5 Effect of weave pattern and side of laminating on peel strength. Construction: warp—2 × 150/34 textured air-entangled polyester 22.4 ends/cm.; fill—12/1 core spun, sheath 100% pimacotton; core—150/34 stretch polyester, 16.8 picks/cm. (Adapted with permission from Haddad andBlack. Journal of Coated Fabrics, Vol. 14, April 1985. c©Technomic Publishing Co., Inc. [8].)

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TABLE 4.2. Effect of Different Filling Yarns on the Peel Strength on VinylLaminated Fabric.∗∗ Vinyl Film Applied to the Side Where Filling Is Exposed.

Peel StrengthN/cm ASTM D

Type of Filling Yarn 751−79

A. Multifilament polyester yarn1. 2 × 150/34 Friction-textured air-entangled polyester 7.92. 2 × 150/34 Friction-textured, plied 2.5 TPI S 9.93. 2 × 150/34 Air-textured Taslan 13.34. 2 × 150/68 Air-textured Taslan 14.9

B. Core-spun yarns1. 17.5/1 Sheath polyester, core 150/34 textured polyester 13.82. 17.5/1 Sheath AvrilIII; core 150/34 textured polyester 17.03. 15/1 Sheath 100% pima cotton, core 150/34 textured polyester 18.54. 12/1 Sheath 100% pima cotton, core 150/34 textured polyester 18.6

C. Spun yarns1. Ring-spun 12/1 3.5TM S high-tenacity polyester 17.02. Ring-spun 12/1 2.75TM S high-tenacity polyeser 14.33. Ring-spun 12/1 3.5TM S crimped polyester 18.04. Ring-spun 12/1 2.75TM S crimped polyester 16.85. Ring-spun 16/1 3.5TM Z regular tenacity polyester 17.06. Ring-spun 18/1 3.75TM S regular tenacity polyester 13.97. Open-end spun 18/1 3.75TM S regular tenacity polyester 23.2

∗ Adapted with permission from Haddad and Black, Journal of Coated Fabrics, vol. 14, April1985, c©Technomic Publishing Co., Inc. [8].Construction: 1 × 3 twill, 16.8 picks/cm. Warp 2 × 150/34 textured air entangled polyester 22.4ends/cm.

Dartman and Shishoo [10] have carried out adhesion of PVC on variouspolyester and polyamide knitted and woven fabrics. Studies on the effect ofmoisture were done by carrying out the coating in an environmental cham-ber with humidity control. Moisture content in the substrate greatly affectsadhesion; environmental moisture, on the other hand, has little effect on adhe-sion (Figure 4.6). They also noted that knitted fabric with a lower cover factorpromotes adhesion.


Resistance to propagation of tear of a coated fabric is of great importancewhere these fabrics are under tension, e.g., in covers, shelters, and architecturalpurposes. If cut or punctured, tear can propagate rapidly under stress, damagingthe material and leading to its failure. Factors controlling tear strength are asfollows:

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Figure 4.6 Effect of adhesion on the moisture content of substrate. PVC-coated woven polyester,5% bonding agents in the coating; D = fabric dried at 105◦C, N = fabric conditioned at 20◦Cand 65% RH, M = wet fabric, N (0%) = same fabric as N but with no bonding agent in coating.(Adapted from Dartman and Shishoo. Journal of Coated Fabrics, Vol. 22, April 1993. c©TechnomicPublishing Co., Inc. [10].)

� construction of the fabric: weave, yarn fineness, and yarn density (Tearstrength is related to yarn strength)

� coating material: formulation and bonding system� adhesion and penetration of coating material on the textile substrate

The effect of various constructional parameters of the fabric on tear strengthhas been reviewed [11]. A study of three woven constructions, viz., matt 3/3,matt 2/2, and plain-weave fabric, shows that tear strength decreases in the orderdescribed due to a lower number of threads at the intersection. Staple fiber yarnhas a lower tear compared to filament yarn. Tear strength decreases with weftdensity of the fabric, but it increases with warp density up to a maximum valueand then decreases. The factors that affect tear strength of uncoated fabric alsoapply to coated fabrics.

Abbott et al. [4] have carried out a detailed study of the tear strength of variouswoven cotton fabrics coated with PVC plastisols by the knife on blanket method.In the uncoated state, the tear strength of different weaves, with the same coverfactor, was found to decrease in the order of basket, twill, and plain weave, dueto the reduced deformability of the structures. Coating resulted in loss of tearstrength in all cases, but the loss of strength varied with the type of weave. Forbasket weave, the loss was ∼70%, in twill ∼60%, and in plain weave ∼25%.Yet, the tear strength of the coated fabric was highest for basket, intermediatefor twill, and least for plain weave, because of their respective tear strength in

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Figure 4.7 Relationship between tear propagation resistance and adhesion (vinyl-coated polyester1000 d Plain weave 9 ends/cm). (Adapted with permission from Mewes. Journal of Coated Fabrics,Vol. 19, Oct. 1989. c©Technomic Publishing Co., Inc. [7].)

the uncoated state. It was also observed that twills and basket weaves madefrom plied yarns had significantly lower tear strength than those made fromsingle yarns. In all cases, the decrease in tear strength is related to reduceddeformability due to the coating.

One of the most important factors controlling the tear strength in a coatedfabric is the adhesion of the coating material on the substrate. Mewes [7] hasreported a relationship between tear strength and adhesion (Figure 4.7). A linearrelationship has been reported by other authors [11,12].

Effect of fabric density on tear strength on PVC-coated fabric has been studiedby Eichert [2]. The tear strength in the warp direction decreases with yarn count,but in the weft direction, no general trend is seen (Figure 4.8). In a trapezoidtear test, an increase in ends and picks increases tear strength. This is due tothe difference in tear geometry in the two tests. In leg tear, the thread systemexposed to testing is not restrained as in the case of the trapezoid test.

The loss in tear strength on coating is much more severe in the weft directionthan in the warp. This is explained by the process of spread coating. Duringspread coating, the coating knife runs parallel to the weft, its dragging actionopens the filament/fibers, leading to greater penetration in the weft. Warp yarns,on the other hand, have much lower crimp due to tension applied during coating,moreover, because the knife runs at right angles to the warp, they are notopened, as such, penetration is lower in the warp [3,13]. Abbott et al. [14] havestudied the mechanical properties, particularly tearing strength, of differenttypes of cotton fabrics by coating with PVC plastisols of different hardnessesand viscosities; polyvinyl butyral and polyurethane. It was seen that the tearstrength is affected primarily by the coating that penetrates the fabric pores, andnot on the nature of coating. No direct relation between viscosity and extent of

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Figure 4.8 Tear strength vs. yarn density in coated fabric. (Adapted from U. Eichert. Journal ofCoated Fabrics, Vol. 24, July 1994. c©Technomic Publishing Co., Inc. [2].)

penetration could be found. Higher tear strength is observed for coating that ismore deformable. In many cases, the penetrated coating was found to be porousand presumably deformable. The use of soft coating alone, or as a base coatwith a hard top coat, showed higher tear strength. A novel way of obtaining hightear strength is to fill the fabric interstices with a water-soluble polymer, suchas carboxymethyl cellulose prior to plastisol coating, followed by leaching thewater-soluble polymer. Thus, high tear strength could be obtained by any typeof coating if the shape of the coating-fabric interface and deformability of thepenetrated coating are properly controlled.

The studies were continued by the authors to investigate the tear strength andpenetration in the coated fabric using different coating techniques, viz., knife onblanket, floating knife, reverse roll, and transfer coating [15]. The tear strengthobtained by knife on blanket and floating knife were found to be similar, eventhough the degree of penetration in floating knife was higher. In transfer-coatedfabric, considerable penetration was noted using a binder coat without gelling,but little penetration occurred when the same was pregelled. The reason forhigher tear strength, in spite of deep penetration, has been explained as due tothe porous, deformable nature of penetrated coating. The factors that determinethe tear strength of the coated fabric are the shape of the coating applicator, thenature of the fabric surface, and the rheology of the coating.


Weathering or degradation of material to outdoor exposure is a complexcombination of various components [16].

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� solar radiation (In sunlight, ultraviolet radiation is the most importantcause of degradation because of its higher energy)

� temperature� humidity and precipitation—liquid or solid� wind� chemicals and pollutants

A coated fabric is essentially a polymeric material. Weathering degrades apolymeric coating by the following process:

a. Volatalization of plasticizer and solvents

b. Rupture of the main macromolecular chain

c. Splitting of the side groups in various ways

d. Formation of new groups and reactions among them

e. Regional orientation—formation of crystalline regions

Studies on the weathering behavior of fibers and fabrics show a rapid degra-dation in most natural and synthetic fibers resulting in loss of strength [16].Attempts to correlate outdoor exposure to accelerated weathering tests havenot been very successful. A typical result is given in Figure 4.9. The degrada-tion of fabric can be prevented by the application of coating. The thicker thecoating, the longer the protection.

Vinyl-coated fabrics are extensively used for covers, tents, shelters, and ar-chitectural use. They are, therefore, exposed to weathering. A study of theirweathering behavior assumes great significance. The weather resistance ofcoated fabric is dependent on the coating composition as well as on the nature

Figure 4.9 Sunlight exposure of wool, cellulosic, and acrylic fibers. (Adapted with permissionfrom “Fibres” by J. H. Ross in Environmental Effects on Polymeric Materials, Rosato and Schwartz,Eds. c©John Wiley & Sons, 1968 [16].)

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of the textile substrate. Strength and elongation are chiefly contributed by thetextile, and the coating protects it from UV radiation during weathering andsubsequent loss of strength.

Krummheuer [17] has investigated the weathering of vinyl-coated polyesterat different locations of varying sunshine hours and different climatic condi-tions. The coating composition was a white plasticized PVC with fungicideand UV stabilizer added. The exposure was done at Miami (U.S.), Dormeletto(Italy), Ebnit (Austria), and Wuppertal (Germany). From the results of expo-sure after five years (Table 4.3), it was found that there was a significant lossof tensile strength and tear strength in thinly coated samples. The loss wasmaximum at those sites where complete coating was lost, i.e., Dormeletto andWuppertal. In thickly coated samples, there was loss of coating, but there wasno major loss of strength. The rate of loss of strength and tear strength is relatedto sunshine hours. The rate of deterioration is very rapid after a substantial lossof coating thickness. The author has suggested that for long-term durability, aminimum coating of about 150 µ is required. The outdoor test does not correlatewith accelerated weathering by Xenotest because pollutants play an importantrole. The effect of a UV absorber is significant for short-time exposure, but forlong-time exposure, their effect is marginal.

Studies were also carried out by Eichert [1] with similar fabric at the samelocations for a period of ten years. The findings are similar to those ofKrummheuer. The loss of tensile and tear strength was found to be more severein the weft direction than in the warp. This is because the fabric warp is locatedat the center and is better protected than the weft threads.


It has been observed that PVC-coated fabric, used for awnings and marquees,shows discoloration on prolonged use. This is more prominent in white orlight-colored fabrics. Such discoloration is due to microbiological attack on thematerial. In PVC-coated polyester fabric, the plasticizer of PVC is susceptibleto microbiological attack, as many of the plasticizers are known nutrients formicrobes. Eichert [18] has studied the microbiological susceptibility of whiteplasticized PVC with and without fungicide. The organisms for study wereAspergillus niger, Penicillum funiculosum, Paecilomyces varioti, Trichodermalongibrachiatum, and Chaetomium globosum. Profuse growth was observed incoated fabric without fungicide but no growth was seen in uncoated fabric andcoated fabric with fungicide. The mechanical properties (tensile strength andtear strength) of the infested fabric, however, did not show any change despitediscoloration. The author suggests a longer test to confirm no loss of mechanicaldamage due to microbiological attack.

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TABLE 4.3. Weathering Test for Five Years of PVC Coated Polyester (Plain-Weave Polyester Fabric 1000 d 9/9 Set Coated withWhite Plasticized PVC).∗∗

Original Coating Thickness of Samples and (Add on g/m2) −0.5% UV Absorber

20 µ (540) 50 µ (600) 230 µ (900)

Locations Res. Thickness Res. Strength Res. Tear Res. Thickness Res. Strength Res. Tear Res. Thickness Res. Strength Res. Tear

Wuppertal 0 37% 22% 0 37% 28% 213 113% 87%Ebnit 22 53% 31% 36 72% 52% 212 87% 94%Dormeletto 0 21% 14% 0 21% 17% 138 109% 101%Miami 13 35% 21% 31 51% 36% 204 97% 139%

∗ Adapted with permission from Krummheuer, Journal of Coated Fabrics, vol. 13, 1983, c©Technomic Publishing Co., Inc. [17].

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For critical applications, yellowing of coated fabrics may be undesirable. InPVC-coated fabric extensively used for architectural application, the yellow-ing can be due to degradation of the polyvinyl chloride due to heat and light.If properly stabilized, the yellowing of PVC-coated fabric may only be dueto the adhesives used in the tie coat formulations. As discussed earlier, thesebonding agents are one-component or two-component systems based on poly-isocyanates that can be aliphatic or aromatic. Aromatic isocyanates are knownto yellow on exposure to light. This aspect has been studied by Eichert [19].White PVC-coated fabrics with different tie coat formulations were prepared,and their whiteness and light transmission properties were measured after ac-celerated weathering. The adhesive system affects the yellowing of the fabric,but the aliphatic isocyanates do not show significant nonyellowing propertiesover many aromatic systems.


1. U. Eichert, Journal of Coated Fabrics, vol. 23, April, 1994, pp. 311–327.2. U. Eichert, Journal of Coated Fabrics, vol. 24, July, 1994, pp. 20–39.3. C. L. Wilkinson, Journal of Coated Fabrics, vol. 26, July, 1996, pp. 45–63.4. N. J. Abbott, T. E. Lannefeld, L. Barish and R. J. Brysson, Journal Coated Fibrous

Material, vol. 1, July, 1971, pp. 4–16.5. E. H. Mattinson, Journal Textile Institute, vol. 51, 1960, p. 690.6. A. P. Harrera, R. A. Metcalfe and S. G. Patrick, Journal of Coated Fabrics, vol. 23,

April, 1994, pp. 260–273.7. H. Mewes, Journal of Coated Fabrics, vol. 19, Oct., 1989, pp. 112–128.8. R. H. Haddad and J. D. Black, Journal of Coated Fabrics, vol. 14, April, 1985,

pp. 272–281.9. R. H. Haddad and J. D. Black, Journal of Coated Fabrics, vol. 16, Oct., 1986,

pp. 123–138.10. T. Dartman and R. Shishoo, Journal of Coated Fabrics, vol. 22, April, 1993,

pp. 317–325.11. Polymer Modified Textile Materials, J. Wypych, Wiley Interscience, New York,

1988.12. Coated fabric, B. Dutta, in Rubber Products Manufacturing Technology, A. K.

Bhowmik, M. M. Hall and H. A. Benarey, Eds., Marcel Dekker, New York,1994.

13. V. K. Hewinson, Journal Textile Institute, vol. 53, 1962, p. 766.14. N. J. Abbott, T. E. Lannefeld, L. Barish and R. J. Brysson, Journal Coated Fibrous

Material, vol. 1, Oct., 1971, pp. 64–84.15. N. J. Abbott, T. E. Lannefeld, L. Barish and R. J. Brysson., Journal Coated Fibrous

Material, vol. 1, Jan., 1972, pp. 130–149.

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16. Fibres, J. H. Ross, in Environmental Effects on Polymeric Materials, vol. 2, D. V.Rosato and R. T. Schwarz, Eds., Interscience, New York, 1968.

17. W. Krummheuer, Journal of Coated Fabrics, vol. 13, Oct., 1983, pp. 108–119.18. U. Eichert, Journal of Coated Fabrics, vol. 24, July, 1994, pp. 77–86.19. U. Eichert, Journal of Coated Fabrics, vol. 24, Oct., 1994, pp. 107–116.

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Rheology of Coating

THE science of deformation and flow of matter is termed rheology. It isconcerned with the response of a material to an applied stress. In coating,

we are concerned mainly with the flow of liquids, solutions, dispersions, andmelts. An understanding of the flow property of the coating material is requiredto control coating thickness, penetration, adhesion, and coating defects.


In order to understand the concept of viscosity, which is the resistance of aliquid to flow, let us consider a situation in which a liquid is confined betweentwo parallel plates—AB and CD. The bottom plate AB is stationary, while theupper plate CD moves (Figure 5.1).

Let the plates be separated by a distance x and the shear force F act tan-gentially on the top movable plate CD of area A, in a direction, so that plateCD slides sideways with a velocity v as shown in Figure 5.1. The top layer ofliquid then moves with the greatest velocity, and the intermediate layers movewith intermediate velocities. The velocity gradient dv/dx through the layeris constant, where dv is the incremental change in velocity corresponding to athickness, dx, of the liquid layer. This term is known as shear rate and is given by

γ = dv/dx (shear rate)

The shearing force acting over the unit area is known as the shear stress.

τ = F/A (shear stress)

Viscosity is defined as the ratio of shear stress to shear rate, i.e.,

η = τ/γ (viscosity)

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Figure 5.1 Flow of liquid under shear: A = area, F = force (dynes).

The unit of shear stress is dynes/cm2, shear rate sec−1, and of viscositydynes − sec/cm2 or poise.

Liquids where shear stress is directly proportional to shear rate are knownas Newtonian. For a liquid, a plot of shear rate vs. shear stress is a straightline passing through the origin. In other words, the viscosity η for a Newtonianliquid is constant, remaining unchanged with rate of shear (Figure 5.2).

Liquids with viscosity that is not constant but varies as a function of shear rateare known as non-Newtonian liquids. The viscosity value obtained for a non-Newtonian liquid at a particular shear rate is known as the apparent viscosity.For non-Newtonian liquids, because viscosity changes with shear rate, only aviscosity profile is capable of expressing the varying viscosity behavior. Thevarious modes of non-Newtonian behavior are given below.


In this type of flow, a certain minimum stress is necessary before flow begins.This is known as the yield value. Once the yield value is reached, the behavioris Newtonian. Mathematically, it can be expressed as τ = τ0 + ηγ , where τ0 isthe yield stress. Examples of material of this type are ketchup and mayonnaise.

Figure 5.2 A Newtonian liquid.

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Figure 5.3 Flow behavior of non-Newtonian liquids: (a) dilatant, (b) pseudoplastic, and (c)Bingham body.


In a dilatant liquid, the apparent viscosity increases with shear rate, i.e., shearstress increases with shear rate. A pseudoplastic liquid, on the other hand, showsshear thinning, i.e., a decrease of apparent viscosity/shear stress, with shear rate.In a dilatant fluid, the dispersed molecules or particles are compressed and piledup, with application of shear, creating resistance to flow. The molecules/particlesin a pseudoplastic fluid arrange themselves in a favorable pattern for flow on ap-plication of shearing force. The different flow patterns of non-Newtonian fluidsand change in apparent viscosity with shear are given in Figures 5.3 and 5.4.

Different mathematical relationships have been put forward to describe non-Newtonian flow behavior. An equation, commonly referred to as the Power lawequation, has been accepted to be of general relevance and applicabilty. Thisequation takes the following form:

τ = K (γ )n

where K and n are constants. In logarithmic form, this law takes the form of

log τ = log K + n log γ

Figure 5.4 Apparent viscosity vs. shear rate of non-Newtonian liquids: (a) dilatant, (b) pseudo-plastic, and (c) Bingham body.

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Figure 5.5 Power law plot of different types of fluids: (a) dilatant n > 1, (b) Newtonian n = 1,and (c) pseudoplastic n < 1.

Thus, a plot of log τ vs. log γ will yield a straight line. If n = 1, the liquidis Newtonian. Dilatant and pseudoplastic materials have n > 1 and n < 1,respectively (Figure 5.5).


There are a number of fluids whose flow properties, such as apparent viscosity,change with time at a constant rate of shear. In some cases, the change isreversible or at least after cessation of shear, the viscosity returns to the originalvalue over time. A thixotropic material may be considered a special case ofpseudoplasticity, where the apparent viscosity also drops with time at a constantrate of shear (Figure 5.6). As the shear force is reduced, the viscosity increasesbut at a lesser rate, forming a hysteresis loop. The area of the hysteresis loop is ameasure of the thixotropy of the coating (Figure 5.7). Thixotropic behavior maybe visualized as isothermal gel-sol-gel transformation of a reversible colloidalgel. Under constant shear, the material undergoes a progressive breakdown ofstructure with better flowability over time.

A common example of thixotropic behavior is the drop of viscosity of paintwith stirring. Thixotropic behavior is advantageous in a coating system becauselowering of the viscosity during coating facilitates application, whereas, higherviscosity at a lower shear rate prevents sagging and dripping.

Figure 5.6 Time-dependent flow: (a) thixotropic and (b) rheopectic.

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Figure 5.7 Hysteresis curves of of fluids: (a) thixotropic and (b) rheopectic.

Rheopexy is the exact opposite of thixotropy in that under steady shear rate,viscosity increases. This phenomenon is observed in some dilatant systems.Such flow is not of much interest in coating.


Rheological properties of the coating material are of primary importance forsuccessful coating. The flow properties of coating compositions are greatly in-fluenced by the shear applied during the coating process, as in calender, roller,blade coating, etc. For use of plastisols (pastes) in spread coating, it is useful toknow the viscosity at high shear and low shear rates. High shear is encountered atthe coating head. A high viscosity at the coating head may cause uneven deposi-tion and may even bend the coating blade. Viscosity at low shear rate and knowl-edge of yield value is also important. A high yield value prevents strike throughin an open-weave fabric, while a low yield value aids leveling of the paste aftercoating [6]. The rheology of plastisol is the most complex and merits discussion.

Very dilute dispersions containing more than 50% plasticizer show New-tonian behavior. Paste formulations, however, have higher polymer loading.As such, pastes show non-Newtonian behavior. Depending on the formulation,they may be pseudoplastic, dilatant, or thixotropic. The flow behavior normallyvaries with the shear rate. A paste may show pseudoplastic flow at low shearrates, dilatancy at moderate shear, and again pseudoplastic at still higher shearrates. It may also show dilatancy at low shear rate but pseudoplastic at moderateshear. Pseudoplastic behavior is due to breakdown of a structure in the pastewith shear, while dilatancy occurs due to peculiar particle size distribution thatdoes not favor close packing, thus resistance to flow.


The viscosity of a simple suspension is given by the well-known Einstein’sequation.

ηs = ηo (1 + 2.5φ) (1)

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where ηs = viscosity of suspension, ηo = viscosity of the suspending fluid,and φ = the volume fraction of the suspended particles. ηs/ηo is known asthe relative viscosity ηr . This equation is true for dilute solutions. At volumefractions >0.025, the ηr becomes much lower than actually observed. Moreover,this equation is applicable when the suspending particles are monodisperse innature, and there is no interaction between the particles and the medium.

The equation cannot be applied to PVC pastes because of the followingreasons:

a. The volume fraction of the suspended particles is quite high >0.2.

b. The suspended particles are not monodisperse.

c. Although paste polymers are resistant to solvation by the plasticizer, slowswelling and dissolution of the polymer particle still occur. Solvation ofthe polymer increases the viscosity of the plasticizer medium, and swellingincreases the volume fraction of the polymer.

Johnston and Brower have developed an equation of apparent viscosity forPVC pastes that is applicable for volume fraction of about 0.2 and for severalresin and plasticizer systems.

log10 ηr = (1.33 − 0.84 φ/φc)(φ/φc − φ) (2)

φc is known as critical volume fraction and is defined as the volume fraction ofthe polymer particle at a stage when it has absorbed plasticizer to the maximumlimit, as in a fairly advanced stage of gelation.

The flow property of the paste and its stability are greatly influenced bythe formulation of the paste. The important factors are particle size, particlesize distribution of the resin, the nature of the plasticizer, and the amount ofplasticizer. Additives also affect paste rheology. The effects of various factorsare given below.


As has been described earlier, paste resins are emulsion grade, obtained byspray drying, of particle size ranging from 0.1–3 µm. These particles are knownas primary particles. However, during spray drying, aggregates of primary par-ticles are formed that can be much larger in size, 40–50 µm. The aggregatesare fragile and break down to primary particles by shearing during paste for-mation. The ease of breakdown depends on the fragility of the aggregate. Thedeagglomeration process also affects the paste rheology.

The paste viscosity depends on the size of the primary particles and thesize and proportion of the aggregate or secondary particles. The resins may becategorized into the following three types:

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(1) High-viscosity resins: the primary particles have size <0.5 µm and aremonodispersed in nature. The secondary particles do not significantly affectviscosity.

(2) Medium-viscosity resins: primary particles are polydisperse, having sizerange of 0.8–1.5 µm. Secondary particles have some effect on viscosity,showing dilatancy at high shear rate.

(3) Low-viscosity resins: they have broader particle size distribution. The sec-ondary particles are larger in proportion and size.

The blending resins have much larger particle size (80–140 µm) than thepaste resins. These resins improve the packing of the nearly spherical emulsionparticles, reducing the interstitial space and lowering the surface area. Thus, fora given plasticizer content, more of it is freely available, lowering the viscosity.

Apart from particle size, there are other factors of the resin that influence theviscosity of the paste. The surfactant used in the manufacture of the resin isretained in the dried resin. The nature and quantity of the surfactant present in theresin is important as it may reduce or increase the paste viscosity depending onits solubility characteristics, with the plasticizer. Moreover, higher temperatureand duration of drying may produce an over-dried resin with reduced plasticizerabsorption. The K value of the polymer does not have much influence on thepaste viscosity.


The viscosity of the plasticizer and its solvating power affect the viscosity ofthe paste. For freshly made paste, the viscosity of the paste varies linearly withthe viscosity of the plasticizer. However, as the paste ages, this correlation islost due to the overriding effect of solvation. The higher the solvation ( δ closerto PVC ∼ 9.7), the greater the viscosity of the paste. Polar plasticizers, such asDBP and TCP, yield highly viscous pastes, with poor viscosity stability, showingdilatancy. For coating purposes, high viscosity and dilatancy are not required.Less polar types such as DOP and DIDP form medium-viscosity pastes withthixotropic properties. As such, they are useful for spread coating. Because oftheir low δ value, dialkyl esters of adipate and sebacates such as DIDA formlow viscosity pastes. An increase of temperature increases aging and viscosity.

The influence of stabilizers, fillers, thickening agents, etc., on paste rhelogyhas already been described (Chapter 1).


As the paste is heated, there is initially a lowering of paste viscosity due to adrop in viscosity of the plasticizer with temperature. This is dependent on thenature of the plasticizer (AB in Figure 5.8).

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Figure 5.8 Change in viscosity during fusion. (Adapted with permission from PVC Plastics byW. V. Titow. c©Kluwer Academic Publishers, Netherlands, 1990 [3].)

A sharp rise of viscosity then occurs mainly due to adsorption of plasticizerby the polymer and due to solution of polymer in the plasticizer-gelation region(BC). The temperature at which the sharp rise occurs is known as the gelationtemperature. As the temperature increases, viscosity increases slowly, showing amaxima (D) at the fusion point. A slight drop in viscosity thereafter, is due to themelting of the microcrystalline structure of the polymer. A solvent immersiontest is useful for determining complete fusion in a coated fabric. The fusiontemperature depends on the nature of the resin and the plasticizer. A paste of ahighly solvating plasticizer and fine particle resin of lower K value have lowerfusion temperature and require lower fusion time.


The process of fluid coating is essentially a fluid in motion, and usual unitoperation parameters can be applied. An overall macroscopic force balance isobtained by the application of the principle of the conservation of momentumto an elemental volume in the fluid. The force acting on the volume is givenby rate of change of the momentum of the fluid surrounding it at any instant,i.e., flux of momentum summed over the entire control surface and the rate ofchange of momentum within the volume.

The net force Fx , acting in say x direction on the fluid element moving withthe velocity of the fluid is a sum of (1) force due to the weight of the volumeelement (body force) Fx B and (2) force due to the stresses acting on it along xdirection, Fx S [Equation (3)].

Fx = Fx B + Fx S (3)

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For an element of a differential mass ρdx dy dz, Equation (3) becomes

ρdx dy dz(dux/dt) = ρdx dy dz g cos β

+ (∂τxx/∂x + ∂τyx/∂y + ∂τzx/∂z)dx dy dz (4)

which, on rearranging, reduces to Equation (5),

ρdux/dt = g cos β + (∂τxx/∂x + ∂τyx/∂y + ∂τzx/∂z) (5)


ux = velocity of the fluid element in x direction

ρ = density of the fluid element

g = acceleration due to gravity

β = angle the fluid element makes with the x axis

t = time

τxx , τyx , τzx = components of stress acting in x direction

By substituting the values of stress and rearranging, we get the NavierStokes equation [Equation (6)]. This is the equation of motion of the elementalvolume in x direction and is used for the analysis of the hydrodynamics ofcoating [7].

ux∂ux/∂x + uy∂ux/∂y + uz∂ux/∂z + ∂ux/∂t

= g cos β − 1/ρ ∂p/∂x + η/ρ(∂2ux/∂x2 + ∂2ux/∂y2 + ∂2ux/∂z2)

+ 1/3 η/ρ ∂/∂x(∂ux/∂x + ∂uy/∂y + ∂uz/∂z) (6)

where ux , uy, uz are velocities of the fluid element in x, y, and z directions;p = pressure generated due to the movement of fluid element in x direction;and η = viscosity of the fluid.

In a blade coating, the substrate to be coated moves under tension, belowa blade. The coating fluid is either poured manually or pumped at the bladenip. The gap between the blade and substrate controls the coating thickness. Astudy of the hydrodynamics of blade coating has been done by Hwang usingthe Navier Stokes equation [8]. In a simple analysis, the coated film thicknessis a function of five variables only. These are the gap between blade and theweb, web speed, viscosity, density, and surface tension of the coating fluid. Themotion of the liquid is considered steady and unidimensional in nature, andthe liquid is incompressible. For motion along x axis, with y axis perpendicularto it, Equation (6) reduces to

∂p/∂x = η ∂2ux/∂y2 + ρg (7)

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Figure 5.9 Blade coating: h is gap between blade and web; x ′ is blade width; at the blade h = 0,u = 0; at the web y = h and u = u0.

The boundary conditions taken are: at the blade, the fluid is motionless, i.e.,u = 0, the fluid velocity at the web is the same as that of the web velocity, u = u0

at y = h. The gap between the blade and the web is h and is small compared tothe blade width x′ (Figure 5.9).

The coater is more like a channel, and a parallel plane flow model is used. Fora Newtonian fluid, by integrating equation [Equation (7)] twice and applyingthe above boundary conditions, the velocity is obtained as

u = u0 y/h + 1/2η (dp/dx − ρg)(y2 − hy) (8)

The total quantity of fluid that passes through the gap per unit length, per unittime, Q is obtained by integrating the above velocity [Equation (8)] betweenthe limits y = 0, u = 0 and y = h, u = u0.

Q =∫ h

0u dy = u0h/2 − (1/12 η) (dp/dx − ρg)h3 (9)

The coating thickness, W, can be obtained by dividing Q by web velocity u0.

W = h/2 − (1/12ηu0)(dp/dx − ρg)h3 (10)

The author has related the pressure gradient to the surface tension of thecoating fluid considering the force balance across a parabolic meniscus at theexit of the blade gap and obtained the relation as

dp/dx = −σ/2h2 (11)

where σ is the surface tension of the fluid.Thus, the coating thickness obtained in terms of the five paramaters men-

tioned above is obtained by combining Equations (10) and (11).

W = h/2 + (1/12η)(σ/2h2 + ρg)h3/u0 (12)

The conclusion of the analysis is that for blade coating, the coating thicknessis half the coating gap, together with a small term that relates directly to surfacetension and gap and inversely to the viscosity and web speed. Hwang has found

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a good correlation between the coating thickness obtained from experimentsand the thickness calculated from Equation (12).

Middleman has carried out analysis of roll, blade, and dip coating for Newto-nian and non-Newtonian fluids [9]. These models have, however, not consideredthe nature of the textile web, which has an important bearing on the coatingthickness.


1. Flow Properties of Polymer Melts, J. A. Brydson, George Goodwin, Ltd. 1981.2. Paint Flow and Pigment Dispersion, T. C. Patton, Wiley Interscience, New York,

1964.3. PVC Plastics, W. V. Titow, Elsevier Applied Science, U.K., 1990.4. Manufacture and Processing of PVC, R. H. Burges, Ed., Applied Science Publishers,

U.K., 1982.5. The Technology of Plasticizers, J. Kern Sears and J. R. Derby, Wiley Interscience,

New York, 1982.6. Spread coating processes, R. A. Park, in Plastisols and Organosols, H. A. Sarvetnick,

Van Nostrand Reinhold, New York, 1972.7. Momentum Heat and Mass Transfer, G. O. Bennett and J. E. Myers, Tata, McGraw

Hill, 1962.8. S. S. Hwang, Chemical Engineering Science, vol. 14, 1979, pp. 181–189.9. Fundamentals of Polymer Processing, S. Middleman, McGraw Hill, New York, 1977.

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Fabrics for Foul Weather Protection


Aperson feels comfortable in a particular climatic condition if his energy pro-duction and energy exchange with the environment are evenly balanced,

so that heating or cooling of the body is within tolerable limits. A core bodytemperature of approximately 37◦C is required by an individual for his well-being. The body maintains this temperature at different work rates and climaticconditions by changing blood flow and evaporating perspiration from the skin.Because the body has a limited ability to cope with the climate, clothing isconsciously selected and adjusted to secure comfort and protection in an ad-verse environment. There are two aspects of clothing comfort, skin sensorial,i.e., mechanical contact with textile surface, and thermophysiological. The ther-mophysiological aspect considers the heat balance of the microclimate createdbetween the skin, air, and clothing, with the external climate and the metabolicheat generated. The routes of heat loss from the body are conduction, radiation,and evaporation. The environmental factors responsible for this heat flow are (1)temperature difference, (2) air movement, (3) relative humidity, and (4) radiantheat from the sun or other sources of thermal radiation. Clothing interacts withthe environment by [1–3]

� thermal resistance—insulation� resistance to evaporation� resistance to wind penetration� structural features, such as, thickness of clothing, clothing weight,

clothing surface area, etc.

The total heat transfer through the clothing of the body with the environment,considering the thermal and evaporation resistance of the clothing, has been

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given by Woodcock:

H = Ts − Ta

I+ Ps − Pa

E (1)

where H = total heat transfer, Ts − Ta = temperature difference between skinand ambient, ps − pa = water vapor pressure difference between skin and am-bient, I = insulation of the clothing, and E = evaporation resistance of theclothing.

The thermal and water vapor resistance are additive, and clothing assembliescan be evaluated by adding the individual values and the intervening air layersbetween them. A person wearing light clothing engaged in light activity in atemperate environment loses about 75% of his metabolic heat by transfer of dryheat. The rest is lost by evaporaton of water from the skin and lungs. As theactivity level rises, perspiration production increases, and the proportion of heatloss by evaporation increases. Perspiring is the main thermoregulatory processat a high level of work rates. If the clothing is impermeable, evaporative coolingcannot occur, and in a hot and humid climate, heat exhaustion may occur. In-sensible perspiration is converted into liquid perspiration below the dew point.In cold climates, wet, perspiration-soaked clothing loses much of its insulationvalue, leading to hypothermia. Moreover, wetting of garments by perspirationgives a clinging appearance and is a burden on motion. Such a situation is acutein garments with fabrics having compact coated PVC, PU, or rubbers. Typicalperspiration values for various activities are given in Table 6.1.

In extremes of climatic conditions, particularly cold weather, apart fromproper thermal insulation, the clothing should be windproof so that cold winddoes not enter into the space between the skin and the clothing, dissipating thewarm air in the vicinity of the skin. Protection against rain, sleet, or snow isalso required, as penetration of moisture in the skin gives a clammy feeling,and its evaporation takes away body heat of the wearer, creating conditionsof freezing. In sum, cold weather clothing, besides insulating, should ideallyhave three main features, it should be water vapor permeable, windproof andwaterproof. Two types of fabrics are in use for foul weather clothing. They are

TABLE 6.1. Typical Perspiration Levels for Various Activities [4].∗

Activity Level Heat Produced (watts) Perspiration Rate (ml/h)

Sleeping (cool dry) 60 80 15 30Walking 5 km/h 280 350 200 500Hard physical work (hot humid) 580 1045 400 1000Max. sweat rate (tolerated for 810 1160 1600

short time)

∗ Adapted with permission from G. R. Lomax Textiles, no. 4 1991, c© Shirley Institute U.K. [4].Conversion factor 1.16 W/m2h = 1 kcal/m2h.

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impermeable coated fabrics and the breathable fabrics. An impermeable fabricis both wind- and waterproof but not water vapor permeable. A breathablefabric, on the other hand, meets all of the features of foul weather clothing andis water vapor permeable.


These fabrics function by blocking the pores of the textile material by acompact polymeric coating that forms a physical barrier to wind and water. Eventhough these fabrics are not breathable, they are comparatively inexpensive, andare widely used for rainwear and foul weather clothing.

Various materials are available for rainwear offering different levels of pro-tection. Generally, single- or double-textured rubberized fabrics are used. Asingle-textured fabric consists of a coating material of natural or synthetic rub-ber coated on one side of a base fabric of cotton, viscose, or nylon fabric. Atypical fabric has a weight of 250 g/m2, with a proofing content of 140 g/m2. Adouble-textured fabric has a rubber coating in between two layers of cotton orviscose fabric. Such a fabric is heavy (400–575 g/m2) but gives excellent protec-tion against rain [5]. By providing proper ventilation in rainwear, it is possibleto transfer condensed sweat outside, minimizing discomfort to the wearer.

Clothing for protection against extreme cold consists of two parts: an innerinsulation material and an outer fabric layer to preserve the insulation fromwind or rain. The outer layers are usually PU-, PVC-, or neoprene-coated fab-rics. PVC- and neoprene-coated fabrics have some limitations in use in extremecold conditions. PVC-coated fabrics have poor low temperature properties andare affected by solvents. Neoprene-coated fabrics, on the other hand, are ratherheavy. Polyurethane-coated nylon is the fabric of choice because of its lightweight, thin coating, and excellent low temperature flexibility [6,7]. Some pop-ular PU-coated nylon fabrics have weights ranging from 100–250 g/m2 witha PU coating between 10–30 g/m2. These fabrics are also used by the mili-tary for various items of cold weather clothing, such as jackets, trousers, caps,and gaiters. Figure 6.1 shows cold weather clothing for the services made ofPU-coated nylon fabric.


The main features of a breathable fabric are depicted in Figure 6.2.Extensive research is being done worldwide to develop fabrics that pro-

vide comfort to the wearer, while offering protection against foul weather.Although poromerics for shoe uppers were developed in the 1960s, the searchfor lightweight breathable fabric for apparel got a fillip from the development

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Figure 6.1 Cold weather clothing ensemble.

of the versatile GORE-TEX® laminates in 1976. Numerous brand productshave been developed and patents filed ever since. In the ensuing sections, anoverview of the technology of these fabrics is discussed.


These fabrics find extensive use in sports and leisure wear. Army personnel onoutdoor duty are exposed to foul weather for days or weeks, especially when onpatrol duty. Breathable fabrics have great application for protective clothing forthe services. Another emerging field for breathable fabric is protective apparelfor healthcare workers, against body fluids and bacterial and viral infections.ASTM has adopted two new specifications to evaluate the barrier effectivenessof such clothing. They are ASTM F 1670-97 and F 1671-97a [8].

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Figure 6.2 Main features of a breathable fabric.

In quantitative terms, a breathable fabric should have the following attributes[4,9,10]:

(1) Water vapor permeability—min. 5000 g/m2/24 h

(2) Waterproofness—min. 130 cm. hydrostatic pressure

(3) Windproofness—less than 1.5 ml/cm2/sec at 1 mbar; measured by air per-meability

Other properties required are as follows:

(1) Durability: tear, tensile, and peel strength; flex and abrasion resistance

(2) Launderability

(3) Tape sealability


A discusssion of the mechanism of water vapor permeability and water re-pellency is useful in understanding the principle of designing breathable fab-ric. Water vapor transport through a fabric/clothing system may occur due todiffusion (driven by vapor concentration gradients) and convection (driven bypressure difference). A discussion of the mechanism of diffusion of water vaporthrough fabric and membranes is given below. Diffusion of Water Vapor through a Fabric

This occurs by the following ways [1,11].

(1) Interyarn space: the diffusion of water vapor occurs through these spacesdue to the water vapor pressure gradient across the two sides of the fabricby molecular diffusion.

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(2) Interfiber space: the contribution of diffusion through a fiber bundle is muchless than the void between the yarns. However, liquid water can permeate thefabric by the wicking action of capillaries of the fiber bundle and subsequentevaporation at the outer surface.

(3) Intrafiber diffusion: in this process, water vapor is absorbed by the fiber anddesorbed at the outer surface. The process is dependent on the nature of thefiber, i.e., hydrophilic or hydrophobic, and is related to moisture regain. It isobvious that diffusion of air and water vapor follow different mechanisms.

A detailed study of diffusion of water vapor through fabric was carried outby Whelan et al. [12]. To arrive at a theoretical model, studies were done usingperforated metal plates of different thickness and hole diameter. It was foundthat water vapor resistance is directly related to the thickness of the plate andinversely to the percent pore area. For plates having constant perforated area,the resistance to water vapor diffusion increases linearly with the diameterof the perforation. An empirical formula [Equation (2)] was derived from theexperimental data.

R = T

β+ 0.71d


β− 1√



where R = water vapor resistance, d = diameter of holes, and β = ratio of voidarea to total area.

Studies of permeation with fabrics revealed that water vapor resistance isrelated to the thickness of the fabric, provided fiber volumes are similar. In thecase of fabrics, because the air path is irregular, the resistance is related to thepercent fiber volume (V f ), rather than percent pore areas found in case of metalplate. An empirical relation derived [Equation (3)] fits the experimental data.

R = 100

100 − V f(0.9 + 0.034V f )T + 0.5 (3)

The results indicate that fabric thickness and the air space in the fabric areimportant parameters for movement of water vapor. The diffusion of watervapor through the pores of a polymer foam structure is related to the thicknessand percent pore volume and follows the empirical equation of the metal plateas given in Equation (2) [13]. Diffusion through Membranes [14]

Diffusion of gases and vapor through a nonporous membrane occurs through adifferent route. The gas is initially dissolved in the exposed polymer surface. Theconcentration built up on the surface is directly proportional to gas pressure, i.e.,Henry’s law. The gas then migrates to the opposite surface under concentrationgradient. The migration of the penetrant can be visualized as a sequence of

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steps during which the molecule passes over a potential barrier separating oneposition from the next. A successful jump requires that a passage of sufficientsize be available, and this is dependent on the thermal motion of the polymerchains. The diffusivity is temperature dependent, which follows an Arrhenius-type expression. For large molecules, the size of the penetrant determines thehole size required. Diffusion rates are higher for good solvents of the polymers,rather than for permanent gases, as they diffuse by plasticizing the polymer.The steady state flux J is given by the expression as

J = DS(p1 − p2)/ l (4)

where D = diffusivity, S = solubility coefficient/Henry’s law constant, l =thickness of the membrane, p1 and p2 are partial pressure of the diffusing gasat two sides of the membrane.

It is common practice to describe the diffusional character of the membraneat equilibrium in terms of a quantity known as permeability. The permeabilitycoefficient is given by P = DS. The structure of the polymer has great in-fluence on the permeability. The factors that increase the segmental mobility,enhance the diffusion rate. Thus, permeation is higher at temperatures higherthan the glass transition temperature Tg . Increase of structural symmetry andcohesive energy of the polymer decreases the permeation rate. The crystallinedomains in the polymer are inaccessible to penetrants and are an impermeablebarrier for the diffusion process. The presence of cross-links reduces the seg-mental mobility, reducing the diffusion rate. The permeation of water vaporvaries from polymer to polymer, depending on the presence of certain po-lar groups (amino, hydroxyl, carboxyl) in the polymer chain that can interactwith water molecules, forming reversible, hydrogen bonds. These groups actas stepping stones for transport of water vapor through the polymer. Water va-por initially absorbed acts as a plasticizer, increasing the intermolecular holes,through which nonhydrogen-bonded water molecules can also pass [11]. Water Vapor Transport through Textiles

The water vapor diffusion through textiles is determined by diffusion testssuch as ASTM-E-96 and ISO-11092. Such tests can lead to erroneous results forhigh air permeability textile materials, because a very small pressure gradientcan produce large convective flows through the porous structure, far outweigh-ing diffusive transport. Therefore, to characterize the potential of a given ma-terial to transport water vapor through its structure, it is necessary to carry outboth diffusion and air permeability tests.

In recent years, extensive work has been done by Gibson on the water va-por transport properties of textiles [15–18]. He has developed a fully auto-mated test method known as “dynamic moisture permeation cell,” that enables

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determination of diffusion and convective properties from the same test, ofdiverse types of materials, such as air-impermeable laminates, very high air-permeable knitted fabrics, woven fabrics, and polymer foams.

In the test method, the sample is placed in a cell. Nitrogen streams consistingof a mixture of dry nitrogen and water-saturated nitrogen are passed over thetop and bottom surfaces of the sample. The relative humidities of these streamsare varied by controlling the proportion of saturated and dry components. Byknowing the temperature and water vapor concentration of nitrogen flows en-tering and leaving the cell, the flux of water vapor diffusing through the testsample is measured. It is also possible to vary the temperature of the cell andpressure drop across the sample. With this test method, Gibson has been ableto measure the following parameters of various materials:

(1) Combined convection and diffusion: these studies are done by varying thepressure drop, across the sample. With a specified pressure drop, transporttakes place both by diffusion and by convection. If there is no pressuredrop across the sample, the transport is only by diffusion. For convectivetransport of water vapor, Gibson has used Darcy’s Law [Equation (5)] forcalculation of permeability.

v = (−kD�p)/µ�x (5)

where v = apparent gas flow, kD = permeability constant, �p = pressuredifference across sample, and �x = thickness. The results on various typesof fabrics clearly show that for high air permeability textiles, convectivewater transport dominates.

(2) Humidity-dependent air permeability: this parameter is important for poroushygroscopic materials that often exhibit humidity-dependent air permeabil-ity due to the swelling of fiber as it takes up water from the environment.

(3) Concentration-dependent diffusion: vapor transport across nonporous hy-groscopic polymer membrane is often highly dependent on the amount ofwater present in the polymer. Studies on several such membranes confirmthe same.

(4) Temperature-dependent diffusion studies: for hydrophilic films and mem-branes, water vapor transport is significantly affected by temperature. Thewater vapor transport is lower at lower temperatures. This effect is importantfor the ability of cold weather clothing to dissipate water vapor during activewear.

(5) Transient sorption and desorption: The method can be used to conducttesting of materials under nonsteady state conditions, such as change inrelative humidity, temperature, or pressure difference across the sample. Inthese transient situations, the variable properties of the material becomevery important, along with factors like sorption rate at which fiber takes upand releases water.

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PB043-06 March 25, 2001 10:50 Char Count= 0 Repellency [19]

A liquid spreads on a solid surface when its surface energy (γS A) is higherthan that of the liquid (γL A). The spreading process lowers the free energy. Thespreading coefficient is given by

S = γS A − (γL A + γSL ) (6)

where γS A, γL A are the surface tension of the solid and the liquid in contact withair, and γSL is the interfacial tension. If the spreading coefficient is positive,spreading occurs, and the contact angle is <90◦. For low energy solids, thespreading coefficient is negative, the liquid is repelled, and the contact angle is>90◦. Critical surface tension of a solid surface is equal to the surface tensionof a liquid that exhibits zero contact angle on the solid. It is a measure of therepellent property of the solid surface. A repellent functions by lowering thecritical surface tension of the solid. For instance, a fluorochemical emulsionlowers the critical surface tension of a nylon fabric to <10 dyne /cm, as such, itcan repel water and hydrocarbon oils having surface tension 72 dynes/cm and∼20 dynes/cm respectively.

A textile surface is not smooth, liquid can migrate into its pores, even thoughthe contact angle on the surface is > 0◦. The transport of a liquid through acapillary is given by

�P = 2γL A cos /r (7)

where �P is the pressure required to force the liquid into the capillary, and ris the pore radius.

Penetration of water into the capillaries of the textile can be prevented byreducing the size of the pores and increasing the contact angle through a waterrepellent treatment. Water repellent finishes are of various types, viz., pyri-dinium compounds, wax and wax emulsions, silicones, fluorochemicals, etc.Several commercial finishes are based on molecules containing polar and non-polar moieties. The polar ends of the molecule attach to the textile, whereasthe nonpolar part sticking outwards repels the water. Silicones are available insolution or aqueous emulsion and are commonly a blend of polymethyl hy-drogen siloxanes and polydimethyl siloxanes. A variety of fluorochemicals areused as repellents, and several brand products exist. Copolymer of fluoro alkylacrylates and methacrylates are primarily used for textiles. Fluorochemicalshave an added advantage in that they are oil repellent as well.

As mentioned earlier, a compact coated fabric is wind- and waterproof butimpermeable to water vapor. The contradictory requirements of water vaporpermeability and waterproofness are achieved, in a breathable fabric, by specif-ically designing the coated fabric to have interconnecting pores of 0.2 to 10 µm.These pores permit much smaller water molecules ∼0.004 µm to permeate from

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inside of the clothing and at the same time prevent ingress of much larger waterdroplets ∼100 µm (for a fine drizzle) to the body. Because the size of watermolecules are of the same order as those of the constituents of air, these microp-orous coatings/films are permeable to air. However, their permeability is so lowthat they impart windproofness to the fabric. Another concept that has emergedrecently are hydrophilic coatings. These are compact coatings that transportwater vapor by permeation through polymeric membranes as discussed ear-lier. Breathable fabric can be engineered from closely woven uncoated textiles,known as ventile fabrics in common parlance.


Breathable fabrics can be categorized into four main types:

(1) Closely woven fabrics with water repellent treatment

(2) Microporous film laminates and coatings

(3) Hydrophilic film laminates and coatings

(4) A combination of microporous coating with a hydrophilic top coat Closely Woven Fabrics

The earliest of the waterproof, water vapor permeable fabrics were the Ventilefabrics developed by the Shirley Institute U.K. [3,11] during World War II. Thedevelopment was an outcome of an urgent need to protect the survivors of aircrew forced to ditch in the cold North Seas from hypothermia. Immersion suitsof Ventile fabric are still in use in the U.K. These fabrics are made from longstaple Egyptian cotton, using low twist mercerized yarns, woven in a denseoxford construction. The fabric weight ranges from 170 to 295 g/m2 for dif-ferent uses. The interyarn pores of the fabric in dry state are about 10 µm.The air permeability is low, but the interyarn spaces and hydrophilic nature ofthe fiber allows adequate water vapor permeability. On wetting the fabric, byrain or immersion in water, the cotton yarn swells, reducing the pore size to3–4 µm. The swollen fabric in combination with repellent finish, prevents fur-ther penetration of water by rain or seawater. The choice of repellent treatmentis critical, as it should still allow absorption of water by cotton yarn to swelland constrict the interyarn pores. The waterproofness of these fabrics is lowand can stand only moderate hydrostatic pressure.

A new generation of high density fabrics made from microfibers of 0.05 to 1 dpolyester, polyamide, viscose, or acrylics has recently emerged as breathablefabric with improved functional properties. The microfibers are obtained bymelt spinning of two incompatible polymers into a single fine fiber, known as abicomponent fiber [20,21]. The cross section of the fiber may be either side byside, core sheath, or matrix fibril type. One of the polymers is then separated bydissolving in a specific solvent, leaving behind microfiber. The yarn is woven

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TABLE6.2. Important Microfibers and Fabrics*

Microfiber Fabric Supplier

1. Trevira finesse Clima guard Hoechst2. Tactel micro Micro spirit I.C.I.3. Dyna bright H2 OFF Toray4. Supplex Dupont

∗ Adapted with permission from M.Van Roey, Journal of Coated Fabrics, vol. 21, July 1999.c©Technomic Publishing Co. Inc. [22].

into various dense fabric constructions like taffeta, twill, or oxford, and given arepellent finish of silicones or fluorochemicals. These fabrics have better waterrepellency than the cotton ventile fabrics and have very soft handle. Some ofthe important microfibers and fabrics are listed in Table 6.2 [22]. Microporous Coatings and Laminates

These are porous membranes laminated to a fabric or porous coating. Thepore size ranges from 0.1 to 50 µm. The most widely used are polyurethanes,poytetrafluoroethylenes, acrylics, and polyamino acids. Among these, poly-urethane is the most popular polymer because of toughness, flexibility of thefilm, and capability of tailor making the property of the film to suit the end userequirement. Various methods of generating microporosity have been reportedin the literature, and these are discussed below [3]. Wet Coagulation Process

Microporous polyurethane coating by direct or transfer process has been dis-cussed in detail under poromerics in synthetic leather (Chapter 7). A film, on theother hand, is obtained by casting, using the transfer coating process on a releasepaper, which is subsequently adhered to the fabric to make laminates. Somecommercial products in this class are Cyclone (Carrington), Entrant (Toray),Keelatex, etc. Incorporation of water-soluble salts in the PU coating solutionhas also been described (Chapter 7). Microporosity is created by leaching thesalts on treatment of the film with water. Products in this class are Porvair,Porelle, Permair, etc. [3,11]. Various improvements in the process have beenreported in the literature. Addition of a water-repellent agent and nonionic sur-factants in the coating solution, imparts water repellency to the pores, resultingin better waterproofness of the coated fabric. A water repellent treatment of thecoated layer further improves the waterproofness [23]. A much higher watervapor permeability and waterproofness have been reported by Furuta et al. [24]by incorporation of about 1% nonporous inorganic filler, e.g., silica (Aerosil)or magnesium oxide of particle size <0.1 µm in the polyurethane resin. Thecoating obtained on wet coagulation shows ultrafine pores of <1 µm, in ad-dition to a honeycomb skin core structure of 1–20 µm pores. The formation

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of these micropores has been explained as due to the subtle difference in therates of coagulation at the resin particle interface. The enhanced water vaporpermeability (>6000 g/m2/24 h) and water pressure resistance (>0.6 kg/cm2)are attributed to the formation of these additional micropores.

The phase separation method for forming microporous coating has beendeveloped by UCB Chemicals, Belgium, for their product Ucecoat 2000 [11].In this process, polyurethane is dissolved in a solvent mixture of methyl ethylketone, toluene, and water, having 15–20% solids and coated on the fabric. Thelow boiling solvent evaporates leading to precipitation of polyurethane in thenonsolvent. The nonsolvent is then removed by drying, to yield a microporouscoated fabric. This process has an advantage over the wet coagulation processin that immersion and washing baths are not required. The number of pores andtheir size in microporous coating obtained by the wet coagulation process are∼106 pores/cm2 and 3 to 40 µm, respectively [9].

A combination of polyamino acid (poly-γ -methyl-L-glutamate) andpolyurethane resin in the ratio of 60:40 to 40:60 has been used by Unitika Co.,Japan, for production of their Exceltech brand of microporous coated fabric bywet coagulation process [25]. An optimum quantity of surfactant is added forimproved water vapor permeability by controlling the porosity. The base fabricused is 70 d taffeta. A scanning electron micrograph shows 107 pores/cm2 inthe product. Because PAU is hydrophilic in nature, Exceltech has higher mois-ture permeability, 8000–12,000 g/m2/24 h, compared to 4000–6000 g/m2/24 hfor microporous PU coating. The other properties are water entry pressure of200 cm and air permeability of <0.07 ml/cm2/sec which are comparable tomicroporous PU coating.

Microporous polyamide coating by wet coagualation process has also beenreported [26]. The process consists of application of an hydrochloric acid solu-tion (5–7.5 N) of polyamide containing 20–30% solids on a textile substrate byknife coating. The coated fabric is immediately dipped into 5–10 N caustic sodasolution bath, to coagulate the polyamide. The excess caustic soda solution isthen squeezed out from the coated fabric by passage through rolls, washed withwater, and excess water is removed. To prevent damage of the fabric by the acidsolution, the neutralization process is carried out quickly. The coating is about20 µm thick, containing two distintict types of pores. The outer layer consistsof small ovoid pores of about 0.02 µm due to instant coagulation, while theinner layer has large elongated pores with average diameter of ∼1 µm. Microporous Polyurethane from Aqueous Dispersions

The process consists of impregnation of a textile web with aqueous disper-sions of polyurethane containing solubility enhancing ionizable groups andcoagulating the polymer by acid or alkali solution, depending on the charge.Such methods are environment friendly as no solvents are used, but the coated

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film has poor adhesion and durability. Dahmen et al. [27] have developed amethod that gives much better adhesion and durability of the coating. The tex-tile is coated with aqueous dispersions of polyurethane having ionic groupsof opposite charge, i.e., both anionic and cationic dispersions. Commerciallyavailable dispersions are taken, and their viscosity is adjusted by the additionof nonionic thickeners to render them suitable for knife coating. Anionic dis-persion is applied as the first coat on the textile, followed by a second coatof cationic dispersion in wet condition, without any intermediate drying orvice versa. The weight ratios of the two dispersions are so adjusted that theanionic and cationic groups are stoichiometrically equivalent. The coated fab-ric is then air dried at about 140◦C, given a flourocarbon treatment, and cal-endered lightly. The process has an advantage in that existing solvent-basedequipment can be used and the time-consuming rinsing process is avoided.The fabric is waterproof and water vapor permeable (water vapor permeability∼2400 g/m2/24 h). Microporous polytetrafluoroethylene [3,22]

Thin films of 5–15 µm PTFE are extruded through a slit die and biaxiallystretched. This results in the formation of microtears of pore size 0.1–1 µm and∼109 pores/cm2 in the film. The film is mechanically weak and therefore lam-inated to textile fabric by adhesives. The film is hydrophobic in nature, and itswater repellency is far superior. GORE-TEX® (W. L. Gore, U.S.) brand productis the most widely used and versatile laminate of this type. The process [28]consists of extruding PTFE paste-dispersion in mineral spirit. The extrudate isdried to form a film of unsintered PTFE. The film is then clamped and stretched.The stretching can be done in one direction (uniaxial stretching) or in two di-rections, right angles to each other (biaxial stretching). The stretching is doneat an elevated temperature, below the melting point of the polymer, at a highrate. While still stretched, the film is heated slightly above the melting pointof the polymer and cooled rapidly in stretched condition. The process gives afilm containing porous microstructure, with a considerable increase in strength.The microstructure of the uniaxially stretched film consists of nodes elongatedat right angles in the direction of the stretch. These nodes are interconnectedby fibrils that are oriented parallel to the direction of the stretch. Typically,the size of the nodes vary from 50–400 µm. The fibrils have widths of about0.1 µm and lengths ranging from 5–500 µm. The development of porosityoccurs due to void formation between nodes and fibrils. When the films arebiaxially stretched, similar fibril formation occurs in the other direction withthe production of cobweb-like or cross-linked configurations with an increasein strength. Porosity also increases as the voids between the nodes and fibrilsbecome more numerous and larger in size. The factors affecting the porosityand strength of the film are as follows:

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Figure 6.3 Production of stretched PTFE film: (1) unsintered film, (2, 3) heated calender rolls, (4)heated roll for heat treatment, and (5) water-cooled roll.

(1) The polymer should have high crystallinity, preferably >98%.

(2) The temperature and rate of stretching: higher temperature and higher rateof stretch lead to a more homogeneous structure with smaller, closelyspaced nodes, interconnected with a greater number of fibrils, increasingthe strength of the polymer matrix. Typically, stretching is done between200–300◦C.

(3) The temperature and duration of heat treatment: during heat treatment,which is done above the melting point of the polymer (350–370◦C), anincrease in amorphous content of the polymer occurs. The amorphous re-gion reinforces the crystalline region, enhancing the strength without sub-stantially altering the microstructure.

Continuous length of the porous film can be obtained by passing the unsin-tered film through the nip of heated calender rolls moving at different speeds.The film is stretched in the gap of the rolls. The extent and rate of stretch de-pends on the friction ratio and the gap between rolls. The stretched film comingout of the calender is passed on a heated roll for heat treatment at ∼370◦C andthen over a water-cooled roll for cooling (Figure 6.3). Nitto Elec Ind., Japan,has come out with a similar product known as Microtex.

Figure 6.4 shows an extreme cold weather suit with an outer GORE-TEX®

laminate and insulation layer. UV/E Beam Polymerized Membranes [29,30]

A rapid method of making microporous films has been developed by GelmanScience, U.S., and patented in 1984 as the Sunbeam process. Monomers andoligomers are cross-linked under a radiation source, UV/E beam and cured inmilliseconds. The polymer is based on acrylates. The pores are of the orderof 0.2 micron, and the film can be very thin, of the order of a few angstroms.The technology permits production at a rapid rate of about 350 ft/min in 1 mwidth and is suitable for making films as well as coating. The film is highlyrepellent and is not wetted by water and most chemicals, only solvents witha S.T. < 18.5 dynes/cm wet the film. The water vapor permeability of thesefilms are ∼1100 g/m2/24 h, and the films can be dry cleaned. These films are

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Figure 6.4 GORE-TEX® suit for cold weather.

marketed as “Repel” and can be laminated on nonwovens for making disposableprotective clothings for a variety of applications such as clean room garments,medical gowns, chemical splash suits, etc. Perforation in Compact Coated Fabric [9,11]

A method has been developed to create micropores of the order of 1 millionpores/m2 by passage of nonporous coated fabric between two electrodes,generating high voltage. The electron beam creates pores through the coat-ing without damaging the fabric. This method gives smooth straight throughperforations. The functional properties are therefore poor.

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PB043-06 March 25, 2001 10:50 Char Count= 0 Extraction of Soluble Component from a Polymer Mixture

A process for making breathable fabric with a microporous polyethylene-tetraflouroethylene (ETFE) layer has been reported by Kafchinski et al. [31].The process consists of making a dope containing a suspension of ETFE ∼0.5–1 µm in an extractable polymeric binder. The dope is cast onto a release paperand dried. The film is next calendered above the flow temperature of ETFE,i.e., about 300◦C, and laminated to a fabric substrate using heated calenderrolls. The binder is then extracted by a suitable solvent which is a nonsolventfor ETFE and does not damage the fabric. This leaves behind a microporousETFE layer on the fabric. Various extractable polymeric binders can be usedsuch as polycarbonate, polymethyl methacrylate, etc. However, water-solublepolymer, such as polyethyl oxazoline, cellulose acetate, etc., are preferred be-cause they can be extracted by immersion in water. The fabrics preferred arepolybenzimidazole, Kevlar,® Nomex®, etc. The resultant laminated fabric isflame resistant as well as breathable. The microporous film can be bonded tothe fabric by adhesive as well.

In another process, fabric is coated with an aqueous-based composition con-taining a film-forming polymer and a suitable proportion of a water-solublepolymer. The coated film is dipped in an aqueous solution of an enzyme, whichdegrades the water-soluble polymer. Washing of the degradation product leavesbehind a microporous coating. The film-forming compositions are emulsions ofacrylics, silicones, polyurethanes, or their mixtures. Water-soluble polymersinclude starch, carboxymethyl cellulose, sodium alginate, etc. The enzymesused are specific for the water-soluble polymers, e.g., for starch it is amylase,and for cellulose derivatives cellulase is used. The water vapor permeabilityobtained is of the order of 4500 g/m2/24 h, which is substantially higher thanthat obtained on washing the coated film with plain water [32]. Hydrophilic Coatings and Films

The mechanism of permeation of water vapor through a nonporous polymerfilm has been discussed. None of the conventional coatings like PVC, PU, andrubbers have the polar groups required for activating the hydrophilic mecha-nism for the transport of water. Although a number of hydrophilic polymers,like polyvinyl alcohol and polyethylene oxide are available, they are highlysensitive to water and would either dissolve completely in contact with rainor swell so heavily that the flex and abrasion resistance would be very poor.A proper hydrophilic polymer for coating should, therefore, have adequateswelling to allow transmission of water vapor and at the same time retain suit-able film strength. Cellulosic derivatives, with a high percentage of crystallinity,lead to stiff coatings. To achieve the proper functional property of the film, anoptimization of hydrophilic-hydrophobic balance is required. The approachesavailable are the use of polymer blends, incorporation of pendant hydrophilic

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groups, or the use of segmented copolymers [3]. Out of these possibilities, useof segmented copolymer, incorporating polyethylene oxide into a hydrophobicpolymer chain has been found successful. The polyethylene oxide segment hasa low binding energy for water, permitting rapid diffusion of water vapor, andit is flexible, so that the end product has soft handle [11,33]. Hydrophilic Polyurethanes

These are segmented copolymers of polyester or polyether urethanes withpolyethylene oxide. The hydrophilicity can be varied either by increasing theoverall hydrophilic content or by changing the length of the hydrophilic seg-ments. A process for their synthesis has been described in a patent to ShirleyInstitute, U.K. [34]. A prepolymer is prepared by reacting polyethylene glycolof molecular weight varying from 1000–2000, with an excess of diisocyanate(preferably 4,4-diisocyanato-dicyclohexyl methane). The prepolymer is chainextended by a low molecular weight diol. Cross-linking can be done using a tri-isocyanate. The polyethylene glycol content is maintained within 25–45 wt.%for optimum properties. An example of this product is Witcoflex [11,33], aseries of coatings manufactured by Baxenden Chemicals, U.K. There are twoproducts, viz., Staycool and Super dry. The coating formulations resemble two-component polyurethane systems. The coating can be applied by direct or bytransfer coating in MEK/DMF solution. The tie coat contains isocyanate cross-linkers. Bion II film of Toyo also falls in this category [22]. Krishnan [8,35] hasreported the synthesis of hydrophilic-polyurethane film based on polyurethaneand dimethyl siloxane. Polyalkylene ether glycol (C atoms in the alkylene groupat least 3) and polyoxyethylene glycol are reacted with a stoichiometric excessof diisocyanate and chain extended by diamine or diols. In a similar manner,polyalkylene ether glycol, polydimethyl siloxane diol, and polyethylene glycolare reacted with an excess of diisocyanate and chain extended. The two systemsare then blended and cross-linked with isocyanates. By controlling the molarratio of the polyols, a proper combination of hydrophilicity and hydrophobicityis achieved by this process. The product is made by Raffi and Swanson and ismarketed as Comfortex. The system is suitable for direct coating as well as forfilm lamination. The laminate can be used for medical protective clothing. Asimilar process has been described by Ward et. al. [36]. The soft segment of ablock copolymer of polyether urethane consists of a hydrophobic componentobtained from either polydimethyl siloxane or polytetramethylene oxide and ahydrophilic part provided by polyethylene oxide. This polymer is mixed witha compatible base polymer, preferably polyether urethane urea. A solution ofthe two polymers in dimethyl acetamide is suitable for coating or for casting asa film. Incorporation of 0.5% LiBr in the coating solution enhances the watervapor permeability. Another approach of preparing hydrohilic PU is by incorpo-rating polyamino acid in polyester polyurethane. The higher the PAU content,the higher the water vapor permeability, but the lower the elasticity of the film

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[37]. Film obtained by transfer coating is laminated on fabric. Excepor U ofMitsubishi is a product of this type [22].

Desai and Athawale [38] prepared a series of PU coating compositionsof varying hydrophilicity by incorporating polyethylene glycol of diff-erent molecular weight in castor-oil-based polyester polyurethane. Thesecompositions were coated on nylon fabric, and their properties were studied.The water vapor permeability showed an increasing trend with an increase ofmolecular weight of PEG. In washfastness test, the weight loss of polymer in-creased with an increase of molecular weight of PEG, indicating that an increasein hydrophilic segments lowers its adhesive strength. In a similar study on castfilms by Hayashi et al. [39], it was found that moisture permeability increaseslinearly with both the concentration of polyethylene glycol and its molecularweight. Setting of the Tg of the polymer at ambient increases its moisture per-meability above room temperature but lowers the same at cold temperature.Such shape memory polymers may find applications in foul weather clothing. Hydrophilic Polyester

Sympatex membrane devloped by Akzo/Enka Germany [40,41] is a hy-drophilic polyester into which polyether groups have been incorporated to im-part hydrophilicity to the membrane. Commercial films, 10–25 µm thick, areproduced by extrusion process. The membrane is colorless and opaque. It hasa water vapor permeability of over 2500 g/m2/24 h, accompanied by about 5%swelling. It is watertight up to 1000 cm. The film is laminated to the fabric.

Hoeschele and Ostapchenko [42] have synthesized breathable waterprooffilm of thermoplastic hydrophilic polyetherimide ester elastomers. The elas-tomeric film is made by the reaction of a diol with a dicarboxylic acid anda polyoxyalkylene imide diacid. The preferred diol and dicarboxylic acid are1,4-butane diol and terephthalic acid. The polyoxyalkelene imide diacid is ob-tained by the imidization of trimellitic or pyromellitic acid with polyoxyalkylenediamine. The diamine has molecular weight between 900–4000 and is obtainedfrom polyethylene glycol and polypropylene glycol. The polyoxyalkylene imidediacid can be depicted by the general formula

Polyoxyalkylene imide diacid: X = polyether chain, R = acid residue

A large stoichiometric excess of diol is reacted with polyoxyalkylene imidediacid and a dicarboxylic acid, in the weight ratio of 0.8 to 3.0. The resulting

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polyetherimide ester elastomer containing 25–60 wt.% of ethylene oxide hasoptimum properties. The elastomer is compounded with UV stabilizers andfillers and extruded as film. The film is laminated to a textile substrate. Watervapor permeability of 3500 gm /m2/24 h as per ASTM E 96-66 has been claimed.

Hydrophilic films and coatings have certain advantages over microporousmaterials [11,33]. In the wet coagulation method, coagulation baths, washinglines, and DMF recovery plants are required. Moreover, precise control over thecoating operation is required to generate a consistent, uniform pore structure,preferably below 3 µm for optimum balance of breathability and waterproof-ness. The entrapment of residues of detergent and sweat into the pores altersthe surface property considerably and reduces the water penetration pressure.Hydrophilic coatings, on the other hand, can be applied by conventional solventcoating equipment, being nonporous, they do not lose their properties on clean-ing. A hydrophilic film is sometimes applied on microporous films to upgradethe water resistance. Thintech of 3M is a microporous polyolefin coated witha hydrophilic polyurethane. Similarly, UCB Chemicals have also developed aproduct in which Ucecoat NPU hydrophilic finish is applied to microporousUcecoat 2000 [22].


A breathable coated fabric is used as an ensemble with a liner. The coatedsurface is always inside, to protect from abrasion. The coating can be on theouter fabric or on the liner. A repellent treatment is generally applied to the outerfabric. Breathable films/membranes are not used as such. They are convertedinto laminates by bonding them to fabric before use. Lamination is done bypowder dot, paste dot, adhesion nets, etc. The resistance of water vapor ofthese laminates depends on the nature and thickness of the membrane, area ofthe membrane covered by the adhesive, and nature of the textile component.There are different methods of making laminates. In a garment, the membraneis always the second layer, from outside, placed directly below the outer fabric.The various types of laminates [9,40] are shown in Figure 6.5.

Figure 6.5 Types of laminates: (a) outer fabric laminate,(b) lining laminate, (c) insert laminate, (d)three-layer laminate, (1) outer fabric, (2) breathable film, (3) insert fabric, and (4) lining material.(Adapted with permission from M. Drinkmann. Journal of Coated Fabrics, Vol. 21, Jan 1992.c©Technomic Publishing Co., Inc.) [40].

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The membrane may be bonded to the outer fabric [Figure 6.5(a)], to theliner [Figure 6.5(b)], on a lightweight knitted material (insert laminates)[Figure 6.5(c)], or may be bonded to both outer and inner fabric into a trilaminate[Figure 6.5(d)]. The type of laminate to be selected depends on the intendedapplication. In the outer fabric laminate, functional aspects are paramount. Theliner and insert laminates give softer handle and better fashion appeal.


The fabric properties of breathable material can be evaluated by B.S. 3546.There are, however, different test methods for measuring water vapor perme-ability. Because the conditions of tests are different, they give different values.A new specification, B.S. 7209-90, has appeared specially for breathable fab-rics [33]. This specification deals with two grades of water resistant, watervapor permeable apparel fabrics. The main requirements of the specificationare (1) water vapor permeability index %; (2) resistance to water penetration asreceived, after cleaning, after abrasion, and after flexing; (3) cold crack temper-ature; (4) surface wetting (spray rating) as received and after cleaning; and (5)colorfastness to light, washing, dry cleaning, and rubbing. The method of de-termining the water vapor permeability index has been discussed in the chapteron test methods.

A comparative evaluation of several breathable fabrics available in trade hasbeen done by Keighley [2]. The fabrics have been categorized into three types,viz., Ventile, PTFE film laminates, and PU coating. Some of his importantfindings are given in Table 6.3. It is seen that cotton Ventiles have low water-proofness but higher permeability. GORE-TEX® laminates have both very highpermeability and waterproofness but are expensive.

TABLE 6.3. Comparative Properties of Different Breathable Fabrics.∗


PTFE LaminatesProperties Cotton Ventile (GORE-TEX®) PU Coating

1. Waterproofness 160 200 >2127 630 >2127hydrostatic head cm

2. Water vapor 4100 5150 4850 5550 2500 4650permeabilityBritish MOD specificationupright cup at 35◦C.g/m2/24 h

∗ Adapted with permission from J. H. Keighley. Journal of Coated Fabrics, vol. 15, Oct.1985.c©Technomic Publishing Co., Inc. [2].

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A comprehensive study has been carried out by several TNO laboratoriesof The Netherlands on the comfort property of rainwear. Rainwear was madefrom waterproof fabrics of different technologies, viz., microporosity, contin-uous impermeable films, and microfiber weaves. In a wear trial, it was shownthat breathable garments caused equal thermal strain as impermeable clothing incold environments but less strain in hot conditions. During hard work, the mois-ture accumulation in breathable fabrics is lower but still large enough to causediscomfort. However, when worn for a whole day, with continually changingmoisture production and climate, a breathable garment dissipates moisture allof the time, whereas in impermeable garments, only accumulation takes place.For extended periods of wear, breathable fabrics give a more comfortable, dryfeeling. Ventilation at appropriate places in the garment has been designed asan alternative means to transport water vapor [43].


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2. J. H. Keighley, Journal of Coated Fabrics, vol. 15, Oct., 1985, pp. 89–104.3. Coated and laminated fabrics, R.A Scott, in Chemistry of the Textile Industry, C. M.

Carr, Ed., Blackie Acad. Professional, London, 1995.4. G. R. Lomax, Textiles, no. 4, 1991, pp. 12–16.5. M. A. Taylor, Textiles, vol. 11, no. 1, 1982, pp. 24–28.6. G. R. Lomax, Journal of Coated Fabrics, vol. 14, Oct., 1984, pp. 91–99.7. C. Cooper, Textiles, vol. 8, no. 3, 1979, pp. 72–83.8. K. Krishnan, Journal of Coated Fabrics, vol. 25, Oct., 1995, pp. 103–114.9. W. Mayer, U. Mohr and M. Schuierer, International Textile Bulletin, Dyeing, Print-

ing, Finishing, 2/1989, pp. 16–32.10. S. Krishnan, Journal of Coated Fabrics, vol. 22, July, 1992, pp. 71–74.11. G. R. Lomax, Journal of Coated Fabrics, vol. 15, July, 1985, pp. 40–66.12. M. E. Whelan, L. E. MacHattie, A. C. Goodings and L. H. Turl, Textile Research

Journal, vol. xxv, no. 3, March, 1955, pp. 197–223.13. G. F. Fonseca, Textile Research Journal, Dec., 1967, pp. 1072–1076.14. Polymer Permeability, J. Comyn, Ed., Elsevier Applied Science Publishers, Ltd.,

New York, 1985.15. Water vapor diffusion in textiles, P. Gibson, Clemson University Coated Fabrics

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Fabrics Conference, Greenville, SC, U.S.A., May 14–15, 1996, pp. 1–28.17. Water vapor transport properties of textiles, P. W. Gibson, Clemson University

Coated Fabrics Conference, Clemson, SC, U.S.A., May 11–12, 1999, pp. 1–28.

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18. P. Gibson, Journal of Coated Fabrics, vol. 28, April, 1999, pp. 300–327.19. Waterproofing and Water Repellency, J. L. Molliet, Elsevier, London, 1963.20. Kirk Othmer Encyclopedia of Chemical Technology, 4th Ed., vol. 10, John Wiley

and Sons, New York, 1995, pp. 654–655.21. J. Hemmerich, J. Fikkert and M. Berg, Journal of Coated Fabrics, vol. 22, April,

1993, pp. 268–278.22. M. Van Roey, Journal of Coated Fabrics, vol. 21, July, 1991, p. 20.23. Y. Naka and K. Kawakami, U.S. Patent 4,560,611, Dec. 24, 1985.24. T. Furuta, K. Kamemaru and K. Nakagawa, U.S. Patent 5,024,403, April 20, 1993.25. T. Furuta and S. Yagihara, Journal of Coated Fabrics, vol. 20, July, 1990, pp. 11–23.26. J. L. Guillaume, U.S. Patent 4,537,817, Aug. 27, 1985.27. K. Dahmen, D. Stockhausen and K. H. Stukenbrock, U.S. Patent 4,774,131, Sept.

27, 1988.28. R. W. Gore, U.S. Patent 3,953,566, April, 1976.29. High Performance Textiles, vol. 9, no. 11, 1989, pp. 7–8.30. E. C. Gregor, G. B. Tanney, E. Schchori and Y. Kenigsberg, Journal of Coated

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Oct. 25, 1994.32. T. Tanaka, T. Tanaka and M. Kitamura, U.S. Patent 4,695,484, Sept. 22, 1987.33. G. R. Lomax, Journal of Coated Fabrics, vol. 20, Oct., 1990, pp. 88–107.34. J. R. Holker, G. R. Lomax and R. Jeffries, U.K. Patent G.B.2,087,909A, 1982.35. S. Krishnan, U.S. Patent 5,238,732, Aug. 24, 1993.36. R. S. Ward and J. S. Riffle, U.S. Patent 4,686,137, Aug. 11, 1987.37. European Pat. Appl. no. 83305387, Sept. 9, 1983, in Journal of Coated Fabrics,

vol. 14, Jan., 1985, pp. 148–164.38. V. M. Desai and V. D. Athawale, Journal of Coated Fabrics, vol. 25, July, 1995,

pp. 39–46.39. S. Hayashi and N. Ishikawa, Journal of Coated Fabrics, vol. 23, July, 1993,

pp. 74–83.40. M. Drinkmann, Journal of Coated Fabrics, vol. 21, Jan., 1992, pp. 199–210.41. High Performance Textiles, vol. 7, no. 6, Dec., 1986, pp. 2–3.42. G. K. Hoeschele, G. J. Ostapchenko, U.S. Patent 4,868,062, Sept. 19, 1989.43. Institute for Perception, TNO report no. IZ F 1986-26, Oct., 1986.

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Nonapparel Coating


A footwear material primarily protects the feet from the environment in whichit is worn. Leather is preferred as a footwear material because it has certain

desirable properties such as high moisture absorption as well as high air andmoisture vapor permeability. The removal of liquid perspiration by evaporationand absorption makes a leather shoe more comfortable to wear. The stretcha-bility of leather makes it suitable for the lasting process, in the manufacture ofthe shoes. The waterproofness of leather is not adequate. A coating to make itwaterproof makes it impermeable. Leather can be given different finishes, viz.,patent, grain, and suede.

The search for synthetic leather has been catalyzed due to periodic shortagesof leather and for economic considerations. Moreover, natural leather comes indifferent sizes and thickness, thus it cannot be handled by automatic, computer-controlled production lines. Synthetic substitutes are now increasingly in usefor various footwear components like shoe uppers, linings, insole, insole covers,stiffeners, etc. A UNIDO report predicts that in the near future, only 60% ofshoe material will be leather, and the rest will be alternate materials. Ideally, aman-made shoe upper material should have a similar appearance, with mechan-ical and physical properties comparable to natural leather. Synthetic materialsfor footwear applications are generally coated fabrics. These may be compactcoated impermeable fabrics or breathable fabric/poromerics.


In the quest for a low cost alternative to leather as a material for shoe, up-holstery, and other applications, vinyl-coated fabrics have been considered themost suitable material. The flexibility of the product can be adjusted by varyingthe plasticizer content, and a leather-like appearance can be given by embossing.The initial PVC fabrics were nonexpanded coatings on closely woven fabrics.

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Figure 7.1 Flow diagram for the production of expanded vinyl.

These fabrics are stiff, lack the handle of leather, and are difficult to upholster.Besides, they tend to harden and crack quickly due to migration of the plasti-cizer. These fabrics typically have a thick skin coat ∼190 g/m2 on plain weavecotton base fabrics of ∼150 g/m2. They find use as upholstery material in lessexpensive outdoor furniture [1,2].

In the 1950s, expanded vinyl-coated fabrics were developed, which had han-dle and drape properties very similar to leather. Expanded vinyl fabric consistsof knitted fabrics as the base material, an intermediate layer of cellular PVC,and a wear-resistant top coat [1,3]. These fabrics are also known as leather clothand are produced by transfer and calender coating. In the transfer coating pro-cess, a thin coating of plastisol is applied on a release paper or steel belt. Thistop coat is partially fused at about 150 ◦C. A second coat of plastisol containing1–5% blowing agent is then applied on the top coat. This layer is again partiallyfused, laminated to fabric, and then subjected to complete expansion and fusionat about 200–230◦C (Figure 7.1). The temperature of the oven depends on thenature of the blowing agent. A glossy top surface can be obtained by applyingan acrylic precoat prior to casting the top coat. Embossing can be achieved byusing embossed release carrier.

In the clandering process, vinyl sheeting containing a blowing agent is pro-duced in the normal manner. The sheet is then laminated to a wear layer and fab-ric. The composite layer is then heated in an oven at high temperature ( ∼200◦C)to activate the blowing agent to produce the foam layer [4]. A cross section ofthe various layers of expanded vinyl is shown in Figure 7.2.

Upholstery grade cloth has a thick foam layer ranging from 360–480 g/m2,a top layer of 180 to 360 g/m2, embossed to a leather-like grain and a knittedbase fabric of ∼100 g/m2. Expanded vinyls are lower in cost and have better

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Figure 7.2 Layers of expanded vinyl.

durability than leather. They have, however, a cool feel, undergo cracking dueto plasticizer migration, and cannot be dry cleaned [2].

Polyurethane-based fabrics are superior to vinyl-coated fabrics in leathersimulation, durability, and low temperature flexibility. These fabrics contain aknitted base fabric, a polyurethane foam middle layer, and a wear resistant topcoat similar to the expanded vinyls [3]. The method of their manufacture isalso similar. A top coat of PU is cast on a release paper in the transfer process.This is followed by a coating mixture of polyol, isocyanate prepolymer, and ablowing agent. The composite is then foamed, cross-linked, and laminated toa textile base. The foamed imitation leather is then separated from the releasepaper [5].

Both expanded vinyl and polyurethane-based leather cloth are widely usedfor upholstery, soft luggages, handbags, shoes, seat covers, and door panels ofcars, etc. [1,3]. An important requirement of upholstery fabric is that it shouldhave proper flame retardant additive to reduce the ignitability of the products,smoke generation, and toxicity of the decomposition products [6].

The vinyls have a very low water vapor permeability. The PU-coated fab-rics, because of their hydrophilic nature, have some water vapor permeability∼5–18 g/m2/h, but the same is not adequate enough for comfort properties ofshoe materials [7].


These are second generation synthetic leathers and are so named as theycontain porous polymers. Besides aesthetics, they have certain other importantcharacteristics:

a. A microporous structure

b. Air and water vapor permeable

c. Water repellent on the outer decorated side

These properties render poromerics as a shoe upper material having propertiesin between leather and vinyl-coated fabrics, because all the properties of leatherhave not been achieved in poromerics as yet.

The most prevalent method of manufacturing [7–9] poromerics is to incorpo-rate a soft porous polymeric mass in the fabric matrix and subsequently coat itwith a porous polymeric layer. The porous polymer is obtained by coagulation

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of a polymer solution in a nonsolvent. The polymer most suited for the purposeis polyurethane. Typically, a solution of polyurethane in dimethylformamide(DMF) is applied on a fabric, by dipping and/or coating, followed by dipping ina large excess of water. The polyurethane coagulates in the nonsolvent due toprecipitation and coalescence. On drying, the microporous polymeric materialis obtained that imparts both waterproofness and water vapor permeability tothe fabric. DMF is used as a solvent for polyurethane because of the following:

� It is a good solvent for PU.� It has a high boiling point, so it does not readily evaporate from solution.� It is highly miscible in water. Water can therefore permeate into the

DMF solution, bringing about coagulation.

The textile materials used as backing are generally nonwoven fabrics thatmay be nonreinforced or reinforced with knitted or woven fabric. Nonrein-forced nonwoven has poor strength and higher permanent set. Fabrics in wovenform are also used in the form of raised pile, however, they suffer from inferiorductility. The fiber properties, particularly its strength, water absorption, andfineness, are important parameters determining the final properties of the endproduct. Fine fibers, particularly microfibers, give excellent aesthetics compara-ble to those of leather [10]. The structure of a poromeric with nonwoven backingmaterial resembles that of leather, the microporous top coat and the backingare comparable to the grain and reticular layers of leather. Poromerics can alsobe obtained by transfer coating on a release paper followed by coagulation andlamination of the microporous film onto a fabric such as in Porvair. Coagulation Process [5,11]

A completely reacted polyurethane solution of about 20% in case of non-wovens or about 10% for a woven substrate is mixed with aqueous pigment,ionic polyurethane (to promote coagulation), and a polyelectrolyte. The solutionis deaerated prior to use. The steps involved in the manufacture are varieddepending on the manufacturer, but a typical flowchart is given below. The fabricis initially impregnated with the PU solution by coating or dipping, followed bycoagulation in a water bath, and excess water removal by squeezing. To the moistPU impregnated substrate, a coating of polyurethane is applied by knife coating.Steam is then passed over the fabric for gelling and to initiate coagulation. Thefabric is then dipped into a coagulation bath containing 15–35% DMF in water.After coagulation, the fabric is washed in water bath, followed by washing insuction drums, where DMF is removed and recovered. The wet fabric is nextdried in a tenter (Figure 7.3).

The pores of the polymer are of micron level and are interconnecting, leadingto permeability. Various factors control microporosity and other properties ofthe poromerics [7,9].

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Figure 7.3 Flow diagram of manufacture of poromerics by coagulation.

(1) The type of polyurethane and the additives

(2) Viscosity and solid content of the solution

(3) Type of fabric

(4) Extent of impregnation

(5) Concentration of DMF in bath and temperature of coagulation

(6) Total dip time and extent of squeezing

(7) Washing efficiency

(8) Drying conditions

A variation of the process is the incorporation of water-soluble [5,7] saltslike sodium chloride or ammonium sulphate into the PU solution. During co-agulation, the salts leach, forming a controlled pore structure. Microporouspolymer can also be obtained by dissolving polyurethane into a volatile solvent(THF) and a high boiling nonsolvent (a hydrocarbon solvent). On evaporation,the volatile solvent leaves behind polyurethane in the nonsolvent, leading toprecipitation of the polymer and coagulation. The microporosity depends onthe quantity of the nonsolvent. A process of coating fabric with porous PUfrom PU dispersion has been developed by Stahl-Holland [10]. A fabric is im-pregnated and coated with aqueous polyurethane dispersion. Precipitation andcoagulation is done at an elevated temperature above 93◦C, using hot water,steam, or microwave. The end product containing porous polymer has muchbetter leather-like properties. The process is nontoxic and requires a simplerproduction unit. Poromerics from Prepolymers [5]

The coagulation process described above uses fully reacted polyurethane.Two methods of manufacture of poromerics from prepolymers have been

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Figure 7.4 Structure of special fiber of Clarino. (Adapted with permission from Encyclopedia ofChemical Technology, Vol. 14, 3rd Ed. 1979, c© John Wiley & Sons [7].)

reported in the literature. They are polyaddition in solution and dispersion.In the former, prepolymer is dissolved with chain extender in a solvent or sol-vent mixture in which the end product is insoluble. As the reaction proceeds, thepolyurethane formed becomes less soluble with time and precipitates out, oc-cluding the solvent. On evaporation of the solvent, a poromeric is obtained.In the dispersion process, isocyanate prepolymer is dispersed in a solvent,(aromatic hydrocarbons) nonsolvent (water) mixture. To this emulsion, thechain extender is added. Polyaddition occurs during evaporation of the solventmixture, leaving behind a porous polymeric structure.

The dried fabric containing coagulated PU is then finished by a thin spraycoating and embossing in a calender, to impart the desired color and grain forleather-like appearance. Suede-like appearance is obtained by buffing.

A number of brand products of poromerics are available in the market[7,8,12]. They differ in the coating as well as in the textile backing. For in-stance, Porvair is a microporous film laminated to a fabric substrate, whereasClarino (Kuraray) contains coagulated polyurethane in a fiber matrix with atop coat of microporous polyurethane. Nonreinforced nonwoven fiber matrixhas been used in Clarino. For this purpose, special fiber has been developed byKuraray that is close in properties to collagen [13]. Two immiscible polymersare melt spun to give a fiber of a cross section shown in Figure 7.4. It hasbeen termed as island and sea fiber where one polymer is dispersed in anotherpolymer matrix. By selective solvent extraction of either the dispersed phase(island) or the matrix (sea), a hollow supple multihole fiber or a bunch of finedenier fiber results. The use of these fibers leads to a product of considerablyenhanced suppleness. Contemporary man-made leather such as Sofrina (Ku-raray) and Ultrasuede (Toray) employ microfibers of less than 0.3 denier forthe nonwoven matrix to obtain excellent properties resembling leather [7].

A cheaper alternative to poromerics [9] is obtained by applying a top coat ofconventional coating of polyurethane by the transfer process, on a fabric sub-strate already impregnated with porous polyurethane by the coagulation pro-cess. This method has much better aesthetics, handle and appearance, and seamholding properties than compact PU-coated fabrics but poorer permeability thanporomerics. These fabrics are extensively used in Europe for ladies footwear.

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The structure of porous films formed by the coagulation process and theeffect of various additives have been studied by Chu et al. [14]. They havenoticed a dense skin layer on top of a spongy base layer. The base layer con-sists of finger-like cavities and large cavities. Cellulose acetate membranes usedfor the desalination process are made by a similar coagulation process usingacetone-solvent and water-nonsolvent and have similar porous structure. Thereasons of formation of such a structure of the cellulose acetate membraneshave been intensively investigated [15–17]. The top skin layer is formed eitherby the evaporation of the solvent or by rapid precipitation at the outermost sur-face. As the water diffuses through the skin layer to the precipitation zone, therate of coagulation slows, and coarser precipitate is formed. Thus, the pore sizeincreases from top to bottom. Similar reason can be attributed to the pore struc-ture of porous polyurethane. The effect of additives studied by Chu et al. [14]are sodium nitrate, water, and defoaming agents. Addition of sodium nitrateand water in the PU-DMF solution accelerates the coagulation process and pro-motes formation of larger cavities in the base layer. Antifoaming agents such asSpan-60 and octadecanol, which are hydrophobic in nature, lower precipitationrate. Dense skin layer is formed with Span-60, and finger-like cavities are absentusing octadecanol. It has also been found that temperature of the coagulationbath plays an important role in pore size formation. A cold bath produces asmall poromeric structure throughout the entire coating; a warm bath creates asmall structure at the surface and a large pore structure underneath. The reasonfor this is the heat of solution generated when DMF and water are mixed. If thebath is cold, it has the ability to absorb this heat without significant temperaturerise. The polymer-rich layer then coagulates quickly, resulting in a small porestructure. If the bath is warm, the water-rich layer penetrates more deeply intothe precipitation layer before the polymer cools to the point that it will coagu-late. This gives rise to a large pore structure. The other important factor in poresize development is the concentration of DMF in the water/DMF bath. A highDMF concentration yields a large pore structure.

7.1.3 POROUS VINYLS [18]

These materials are used as liners in footwear for absorption of moisture. Theyare much cheaper than PU-coated fabrics and poromerics. There are varioustechniques to produce these materials:

(1) Incorporation of soluble material into the plastisol matrix, calendering itinto sheet form, leaching the soluble material to produce voids, and thenlaminating it on to fabric

(2) Foaming (chemical or mechanical)

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(3) Sintering—careful heating and pressure sinters solid vinyl particles,generating a solid containing voids

Murphy [18] has described a novel and improved process of manufacturingabsorptive vinyl. The process consists of coating a fabric with plastisol contain-ing noncompatible thermoplastic polymer particles. During fusion and gelling,the incompatible thermoplastic softens and shrinks, as it has no adhesion withthe vinyl matrix. By controlling the size and content of particles and by applyingmechanical stress, it is possible to create interconnecting tunnel-like voids. Theabsorptive vinyl is laminated to a backing material of natural-synthetic fiberblend and finished. Absorptive vinyls have high moisture absorption and des-orption rates and are particularly suited for liner applications using impermeableuppers.

7.1.4 PTFE LAMINATE [19]

GORE-TEX® is a hydrophobic polytetrafluoroethylene film containing mi-cropores. It repels water but permits passage of water vapor. A triple laminatewith an outer layer of textured nylon fabric, a middle layer of GORE-TEX®

film, and an inner layer of knitted fabric can be used as a shoe upper. Totalingress of water is prevented by sealing rather than stitching the seams forconstruction of the shoe. These uppers are used for applications where a highdegree of water repellency combined with breathability are required.


The use of shelters made of textile material for protection against the ele-ments dates back to mankind’s earliest days. Over the years, different typesof tents have been developed to meet various requirements of the military, ex-plorers, nomads, etc. These tents are made of cotton canvas with wax emulsiontreatment to provide water repellency. The development of high strength, rotproof, hydrophobic synthetic fibers along with improved polymer coating hasgiven an impetus in the use of coated fabric as a membrane material to envelopevery large building structures of the size of stadia or airports.

The fabric envelope is capable of resisting the elements of weather such aswind, rain, snow, sunlight, and even biological degradation. Although lowercost transparent film can be used as a membrane material, it lacks durability.Coated fabrics are the most widely used material because of high strength andenvironmental resistance. The advantages of these fabric envelope buildingsare summarized below [20,21].

� The coated fabric envelope is much lighter than conventional buildingmaterial (may be 1/30th) requiring much less structural support andreinforcements. This reduces the cost of the building.

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� It provides large obstruction-free spans, suitable for large gatherings.� The construction time is much shorter.� Smaller envelopes can be dismantled and reerected elsewhere.� Fabric envelopes are better resistant to natural hazards like earthquakes.


The fiber and the fabric requirements of the coated fabric envelope are quitestringent. They should have the following properties:

a. High strength, to withstand the tension applied during construction of thestructure, weight of the suspended fabric, and stresses due to wind, rain, andsnow

b. The fiber should be creep resistant and have high modulus, for dimensionalstability and resistance to deformation.

c. Retention of mechanical property in widely varying temperature conditions

d. Resistant to water, sunlight, and atmospheric pollutants

e. Long life

f. Low cost

The cost factor eliminates high-performance fibers such as aramids and car-bon fibers. The choice is thus restricted to high tenacity polyester and glassfiber, out of which polyester is more popular. Continuous filament yarns arepreferred over staple yarns because of higher strength and resistance to exten-sion. The twist level is kept low to prevent fiber slippage and yarn rupture.Usually, woven structures are preferred for rigidity and dimensional stabil-ity. The type of weave should be such as to produce good yarn packing tominimize fabric deformation under tension and to provide a certain level ofresistance to water and wind penetration. Generally, plain weave and 2 × 2basket weaves are used. The high tenacity polyester fabrics used for archi-tectural purposes have been categorized into a few fabric types, with fabricweight ranging from 220 to 630 g/m2. A typical fabric is made of 1000 dyarn in plain weave (9.5 × 9.5 ends × picks/cm) having a weight of 220 g/m2

[20,22].The coated fabrics are orthogonally anistropic, i.e., they elongate differently

in warp and weft directions. This aspect is to be taken care of in designingtextile structures. Weft inserted warp knit and multiaxial knits have been triedto achieve isotropic structures, but they have not been successful, because oncoating, the former develops anisotropy, and the latter results in a very stiffmaterial [22].

Glass fabric has high strength, resists stretching, does not wrinkle, does notburn, and has high reflectivity, keeping the interior of the structure cool.

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The polymer used for coating or laminating, the architectural fabric, shouldimpart certain important properties to the membrane fabric. These propertiesare as follows [20,23].

� waterproofness� impermeability to air� resistance to abrasion and mechanical damage� resistance to weathering and pollution� ability to transmit and reflect light (It should have adequate translucency

to provide natural illumination in daylight hours.)� weldability� flame retardance

Such requirements are met by two polymers, they are PVC and polytetrafluo-roethylene (PTFE). Coating is preferred to lamination because laminated fabricstend to delaminate on repeated flexing and wind lashing. Because of the highsintering temperature of the polymer, PTFE can only be applied on glass fabric.Two materials have found success as membrane material for envelope, PVC-coated polyester and PTFE-coated glass fabric [20,21,24]. A third material,silicone-coated glass fabric, is also emerging as a material of choice.

PVC-coated fabrics exhibit a dirt pick up problem, which is usually mini-mized by applying a thin top coating of polyurethane or acrylic lacquer. Whiteis considered the color of choice, because the high reflectivity reduces surfacetemperature and enhances service life [3]. Translucency of the fabric is achievedby controlling the yarn density and by properly formulating the PVC compound.PVC-coated fabrics are easily joined by welding and are easier to handle forconstruction of the structure. A disadvantage of PVC is its slow embrittlementdue to gradual loss of plasticizer. However, a properly designed fabric has a lifeof over 15 years. Different grades of these fabrics are available in the trade tomeet the varying load-bearing capacities. The fabric weight ranges from 600 to1000 g/m2. These fabrics, because of their inherent flexibility, are suitable forsemipermanent structures that may be taken down and reerected on a new site[20,22,25].

PTFE-coated glass fabric has several advantages. The polymer is chemicallyinert, self-cleaning, highly resistant to weathering, inherently translucent, andhas excellent flame retardant properties. There are, however, certain problemsin using this fabric. The abrasion resistance of PTFE is poor, the fabric is brittlecausing handling problems, and joining of the fabric panels is difficult. The abra-sion resistance is improved by incorporating glass filler in the outermost layer ofthe coating. The brittleness of glass fabric is reduced by a finishing treatment.Hot-melt PTFE resin is used for joining the fabric panels. Different gradesof fabric are available with fabric weight ranging from 1250 to 1450 g/m2.The fabrics have a solar reflectance of about 70% and transmission of about

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15%. In view of the durability and stiffness of the fabric, they are used forpermanent structures, that have a life of over 20 years [23].


Structures using coated fabric envelopes can be classified into three maintypes. They are tents, air inflated structures, and tensile structures. As statedearlier, wax-emulsion-treated canvas has been in use as tent cover for a longtime, however, they have now been replaced by PVC-coated polyester. In atent, the fabric is draped on a frame and is not tensioned. In large structures,the fabric is tensioned by air inflation or cables [20]. Air Supported Structures and Shelters

In these structures, the fabric assembly that serves as the roof is anchoredand sealed to a ground foundation. Air is pumped inside to inflate and tensionthe envelope (Figure 7.5). The pressure required is only 3% above the ambient,therefore, it does not affect the comfort of the occupants. The entrance andexit doors are air locked to minimize drop of pressure when opened. Any fallof pressure is automatically corrected by pumping in air by a compressor. Adouble-layer roof is used to provide thermal insulation. Air houses were ini-tially developed for protection of radar antennae and telecommunication equip-ment from high velocity winds and weather. The material used for radomesare neoprene- or hypalon-coated glass for transparency to electromagnetic

Figure 7.5 Air-inflated structure. (Adapted with permission from R. J. E. Cumberbirch. Textiles,Vol. 16, no. 2. 1987 c© Shirley Institute, U.K. [21].)

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radiation. Air supported buildings are now used for sports halls, exhibitionhalls, mobile field hospitals, storage bases, swimming pools, etc. For these pur-poses, the fabric widely used is PVC-coated polyester. These types of buildingsare very stable, because highly flexible fabric distorts to distribute the damagingload due to strong winds and returns to the original shape after the wind hasabated. The response to wind buffeting can be changed by varying the internalpressure using an automatic control system related to wind speed [21,24,25].

Another way of supporting the membrane structure is to use air beams. Airbeams are convex, air-inflated (30–70 kPa) support tubes, which can be up to 1 min diameter. They are mainly used by the military as lightweight shelters [20]. Flexible Barrier Storage System

Defense equipment and weapon systems, if left unprotected from weather,while not in operation, may undergo corrosion and microbiological degradationdue to uncontrolled humidity and condensation of moisture due to temperaturefluctuation. This may lead to operational failure and delays in reactivation ofthe equipment. Preservation by surface coatings such as paint and grease is nota long-time solution. After extensive experimentation, it has been found thatthe equipment could be preserved indefinitely in a controlled humidity between30–40% relative humidity.

Flexible barrier storage systems are used by defense forces for preservingmilitary equipment like tanks, helicopters, aircraft, and weapon systems fromcorrosion, rot, mildew, insects, dust, pollutants, and UV degradation. The equip-ment is mounted on a baseboard and is completely covered by reusable flexiblebarrier or shroud. Proper sealing is done by zip fasteners and Velcro to makethe enclosure airtight. The cover is connected to a portable, solid desiccant-type dehumidifier through a flexible duct. Dehumidified air is circulated in theenclosure to maintain a relative humidity between 30–40%. There is no pres-sure difference between the inside and the outside of the envelope. Inspectionwindows and visual detectors are provided to monitor the relative humidityinside. An alarm system is also available in case the RH alters from the desiredvalue. Several hardware systems can be connected to a single dehumidifier bya manifold. With this system, the equipment can be preserved for very longperiods and can be reactivated in a very short time, enhancing the operationalefficiency. Another major advantage is the flexibility of choice of storage sites.An alternative to the dehumidification process is to evacuate the enclosed en-velope. However, with the vacuum system, it is difficult to maintain 30% RHcontinuously. Besides, the barrier material clings to the object forming creasesleading to rupture and air leakage. A flexible barrier system showing protectionof a tank is shown in Figure 7.6.

The barrier material should be lightweight, strong enough to resist environ-mental stresses, waterproof, flame retardant, and should have low water vapor

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Figure 7.6 Flexible barrier system for protection of tank. Courtesy M/S Arctic India Sales, Delhi,India.

permeability, below 1.2 g/24 h/m2. Different materials are used in the trade,they are butyl and PVC-coated nylon or polyester fabric. However, calenderedmultilayered PVC film is also used. The life of a shroud is between 7–9 years. Tension Structures

In these structures, metal pylons or frames and tension cables are used tosupport the fabric. The fabric is tensioned by cables attached to the fabricby clamps. The tensioned structures are curvilinear and may be paraboloid orhyperbolic-paraboloid in shape. The curvature and prestress due to tension-ing resists externally applied loads. To maintain rigidity and stability of thestructure, multidimensional tensioning with properly designed curvature is re-quired. These structures are used for permanent buildings and use generallyPTFE-coated glass fabric [20,21,24].

Major challenges in the construction of these structures are their rigidityand stability to high velocity winds, rain, and snow. The designing, therefore,requires a knowledge of textile, engineering, and architecture. There are about150 structures all over the world, which have been erected within the last twodecades. These include airport terminals, stadia, and department stores. Thelargest being the Haj terminal building at Jeddah International Airport for whichabout 500,000 m2 of PTFE glass fabric was used. These buildings have a distinctstructural identity, the diffuse natural light through the roof is gentle to the eye

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TABLE 7.1. Properties of Awning Fabrics.


Vinyl-Laminatedor Vinyl-Coated Polyester Vinyl Coated Acrylic-Painted 100% Acrylic

Properties Backlit Cotton Polyester/Cotton Woven

1. Base fabric Polyester Cotton Polyester/cotton Solution dyed acrylic ormodacrylic

2. Coating / Vinyl/acrylic Vinyl Acrylic Fluorochemical finishfinish

3. Wt, g/m2 540 740 500 400 450 300 3304. Width, cm 155 180 78 78 1555. Colors, top Several solids Several solids Several solids Several solids

and stripes and stripes and stripes6. Colors, White/clear Pearl gray Pearl gray Same as top

underside7. Opacity Translucent Opaque Opaque Opaque8. Backlit Yes No No

translucency9. Durability, 5 10 5 5 5

years10. UV resistance Yes Yes Yes Yes11. Mildew Yes Yes

resistance12. Water Yes Yes Yes Yes

repellency13. Flame Yes Yes No Modacrylic yes

retardancy Acrylic no

Compiled from Reference [26].

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and gives a delightful ambience. The high translucency and strong membranescould one day permit games like soccer to be played under cover on real turf.Development work is going on to construct membrane structures to aid cropproduction in desert regions [20,24].


These are architectural projections of a building. They provide shade, weatherprotection, decoration, and a distinct identity to the building. Awnings arewholly supported by the building to which they are attached by a lightweightrigid or retractable frame over which awning fabric is attached. Canopies aresupported from the building as well as from ground. Illuminated or backlitawnings offer high visibility to commercial buildings. This is achieved by at-taching lighting to the frame beneath the fabric cover. The awning and canopyfabrics are similar, however, backlit awnings are more translucent. Some of thedesirable properties of awning fabrics are as follows [20]:

� resistance to ultraviolet radiation� flame retardancy� mildew resistance� cleanability

From the various types of commercially available awnings listed in the“Awning fabric specifiers guide,” it is seen that the major types of fabrics usedfor awning and canopies are as follows:

� vinyl-laminated or vinyl-coated polyester-backlighting� vinyl-coated cotton� acrylic-painted cotton or polyester cotton� 100% acrylic woven

Out of these, vinyl-laminated or vinyl-coated polyester-backlighting is themost popular. Some important features of these classes are given inTable 7.1.


The low permeability of certain polymeric coatings have made coated fabricsa material of choice, as lightweight flexible containers for both gases and liquids.The containers for gases are known as inflatables and are meant to providebuoyancy. The liquid containers are used as storage vessels for fuels, water,etc. [27].

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These are designed to contain air or carbon dioxide and are meant for buoy-ancy applications. One of the important uses of inflatables are in lifesaving aidssuch as in life jackets and in emergency rafts. Life jackets can be inflated byair, or automatically by carbon dioxide cylinder in case of an emergency. Thebuoyancy is properly designed so that the wearer is held in correct position inwater. Modern life rafts are designed with capacities of 40 persons. They havetubular multiple buoyancy chambers, so that they remain afloat even if acciden-tally punctured. Emergency life rafts are inflated in seconds, by solid/liquefiedcarbon dioxide. All the rafts are fitted with canopies for protection againstweather. The canopy usually has a fluorescent orange color for easy detection.The floor of the raft should be watertight and provide insulation from seawater.These rafts are made from nylon fabric (130 g/m2), coated on both sides withpolyurethane. Some naval rafts use nylon laminates containing an intermediatelayer of butyl rubber, which has low permeability to gases [27,28].

Inflatable crafts are used also for patrol duty by the Coast Guard or for leisure.They are inflated by air at pressure between 15 to 25 kPa and may be propelledby oars or a small overboard motor (Figure 7.7). The air cushion of a hovercraftis also contained in a coated fabric skirt fitted around the hull at a pressure of3 kPa. The lift to the craft is propelled by a downward air flow.

An interesting application of an inflatable is for constructing temporarybridges for ferrying heavy military vehicles across a river or pond. This is

Figure 7.7 A motorized raft. Courtesy M/S Swastik Rubber, Pune, India.

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done by laying a set of floats across the river. The floats are air inflatable buoys,containing tubular, multiple buoyancy chambers made of coated fabric. A typ-ical float fabric is a flexible composite, consisting of two layers of nylon fabricwith neoprene coating in the intermediate layer as well as on both outer layers.The weight of base fabric and coated fabrics are ∼240 g/m2 and ∼1500 g/m2,respectively. Other applications of inflatables include oil booms to contain oilslicks in the sea and air bags for manipulation of awkward heavy objects overwater in offshore industries. Deflated air bags are used in the salvage operationof sunken objects, like crashed aircraft from sea by inflation [27,28].

Hot air balloons for leisure are also inflatables that are made of lightweightnylon fabric (30–60 g/m2) in rip-stop construction, coated with a thin layerof PU to reduce porosity. However, due to the temperature of hot air inside(∼100◦C), UV exposure and manhandling, the fabric can last for only 300–500flight hours [28].

The main components of inflatables are (a) coated fabrics, usually calen-dered; (b) the adhesive; and (c) the inflation valve. The factors that are to beconsidered for designing inflatables, particularly crafts for marine applications,are as follows [29]:

(1) Strength of the coated fabric: the inflatables may be considered as a thin-walled cylinder. The strength requirement of the inflatable is obtained fromthe hoop stress, exerted due to pressure of inflation; i.e., pD/2t (where p =pressure, D = diameter of the tube, and t = thickness), along with a suitablesafety factor. Usual inflation pressure is about 15–20 kPa. The strength ofthe coated fabric, tensile and tear, is obtained from the textile substrate anddepends on type of yarn and construction. Normally, nylon or polyesterwoven fabrics are used.

(2) The polymer coating: this is dependent on the type of use and life expected.The polymers generally used are neoprene, hypalon, and polyurethanes.

(3) Airtightness: the polymeric coating should be pinhole-free for proper gasholding properties.

(4) Resistance to weathering and UV degradation: this is required as the inflat-ables are used outdoors for very long periods.

(5) Resistance to abrasion: a higher resistance to abrasion is required for pro-tection against damage due to launches and landings on sandy and rockyshorelines.

(6) Resistance to oil and seawater: these are important requirements as thecrafts are used in marine and oily environments.

(7) Flexibility: the fabric should retain its flexibility over a wide range of tem-peratures (−30 to 40◦C). Low temperature flexibility down to −45◦C isrequired for rafts inflated by solid carbon dioxide.

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The coated fabrics for inflatables are produced by calendering or spreading.However, calendering is more suitable for compact, thick polymeric coatingson heavy-duty base fabric. The coated fabric, which usually has a width of∼150 cm, is cut into panels of various shapes depending upon the shape of thefinished article. The edges of the fabric to be adhered are buffed and joined by acold curing adhesive to form the inflatable structure. The adhesive should havecertain important characteristics. They are compatibility with the polymer, goodgreen tack, rapid cure, resistance to oil and water, and appropriate cure strength.


Coated fabrics are particularly suitable for collapsible storage containersand for transporting liquids by land and sea. Dracone barges, which are largeflexible containers, are suitable for towing liquids on the ocean. One of thelatest applications of these are in pollution control due to oil spillage in thesea. The spilled oil is contained by a boom system placed around the slick.The oil is then pumped into dracones and transported away for reclamationor incineration. Dracones can also be used to transport the large quantity ofdetergent required to disperse the oil spill [27]. Collapsible containers are usedas fuel tanks for military aircraft and as fuel containers in temporary air fields.The coated fabric can be heavy, about 5000 g/m2 on a nylon base fabric ofabout 600 g/m2. The polymer used is generally neoprene. However, for storageof fuels, nitrile rubber is used, and an approved grade of PU is used for storageof drinking water [28].

A detailed investigation has been reported to find out the parameters of weaveconstruction by which tear resistance of base fabric used for large collapsiblefuel tanks (∼200 kl) could be maximized. High-tenacity nylons of 840 d and1050 d were woven into plain, twill, and basket weave designs with differentyarn densities. Considering both tear resistance and dimensional stability, a11 × 11 ep/pp cm, 2 × 2 basket-weave fabric, constructed with 1050 d yarngave best overall performance [30].


Tarpaulins are used as covers to protect commodities from damages dueto weather. In the agricultural sector, they are extensively used for protectinggrain and machinery, while in the construction industry, they are used to protectbuilding supplies like timber and wet concrete. In transportation of goods byroad, they are used to cover the cargo. Traditionally, canvas covers made fromheavy-duty cotton fabrics with a wax emulsion and a rot-proof treatment havebeen used for this purpose. However, canvas covers have now been completelyreplaced by 450–500 g/m2 PVC-coated fabric because of their lighter weight and

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inherently water- and rot-proof nature. Some important properties of tarpaulinsare (1) waterproofness; (2) strength; (3) tear, puncture, and abrasion resistance;(4) flexibility at a wide range of temperatures; and (5) durability.

The coated fabrics used are vinyl-coated nylon or polyester fabric withweights between 500–600 g/m2 (∼350 g/m2 coating). For covers likely to becontaminated by fuels/oils, neoprene, hypalon, or PU-coated fabrics are used[31]. High density polyethylene woven fabrics laminated on both sides by lowdensity polyethylene films are also being used as covers. For military use,tarpaulins should have camouflage properties as well. A desirable feature forvehicle covers is reversibility that would cater to change of terrain, e.g., fromgreen belt to desert terrain.


Untill recently, safety belts have been the only protection for passengers ina car crash. During the last decade, air bags or inflatable restraints have gainedsignificant importance as protection for the driver and passengers in case of acollision. The original bag was designed for head-on collision, but at present,side impact bags, knee bolsters, side curtain, etc., are available for safety in anytype of crash. Because frontal collisions are a major cause of accidental deaths,air bags are being introduced as a standard item in vehicles by legislation inthe U.S. Consumer consciousness coupled with legislation has resulted in rapidgrowth of air bags during the last decade [20,32,33].

The air bag is built into the steering wheel and the instrument panel of the car.An air bag module consists of the air bag, crash sensors, and mounting hard-ware. In case of frontal collision equivalent to 20 km/h against a wall, sensorsset off the igintor of the inflator. Pellets of sodium azide in the inflator igniteand release hot nitrogen gas. The gas passes through a filter to remove ash andother particulate matter and inflates the air bag. A pressure of 35–70 kPa isgenerated. The air bag is fully inflated within 60 ms and cushions the occupantfrom impact. After absorbing the forward force, the air bag deflates after 120ms [20]. The capacity of a driver-side air bag is normally ∼65 L, but for smallercars, it is ∼35 L. Passenger-side air bags are much larger (100–300 L) [33].

The air bag fabric is made from nylon 66 because of its high weight-to-strength ratio and is preferred over polyester because of higher elongation,allowing the force to distribute widely. The driver-side air bag has an elastomericcoating to provide heat shielding and ablative protection to the fabric from thehot gases. Moreover, coating seals the fabric pores and permits precise controlin the deployment of the air bag.

The coated fabric should be antiblocking, have high tensile and tear strength,good adhesion, and long-term flexibility to cyclic temperature changes betweenextreme cold and hot conditions (−10 to 120◦C). Besides, the fabric should

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TABLE 7.2. Neoprene-Coated Fabrics for Air Bags.*

ConstructionPlain Weave Base Fabric, Coated fabric,

Nylon Yarn d Ends/cmx Picks/cm wt. g/m2 wt. g/m2

840 10 × 10 190 280420 18.4 × 18.4 185 260

∗ Adapted with permission from E. T. Crouch, Journal of Coated Fabrics, vol. 23, Jan. 1994.c© Technomic Publishing Co., Inc. [32].

be soft and smooth so as not to cause secondary abrasion or bruises and havegood packageability. Untill recently, two types of neoprene-coated fabrics wereused because of better environmental stability and flame retardant propertiesof neoprene [32]. One is a heavy fabric made of 840 d nylon and the other alighter fabric woven from 420 d nylon. The details are given in Table 7.2.

The need to enhance the life of the air bag and further reduce the size ledto the development of silicone-coated air bags. Silicones are chemically inertand maintain their properties for a long time at temperature extremes. An agingstudy of both neoprene- and silicone-coated fabrics was carried out at 120◦Cfor 42 days. The elongation of the fabrics prior to aging were about 40%. Afteraging, the elongation of silicone-coated fabric was 32% but that of neoprene-coated fabric dropped sharply to only 8%. This has been attributed to the poorcompatibility of neoprene with nylon. It is possible that chlorine in the neopreneproduces an acidic environment, embrittling the nylon fabric. Another drawbackof neoprene-coated material is that it should be dusted with talc to prevent self-adhesion, which creates dust in the vehicle interior following deployment ofthe bag. Silicone-coated fabrics are more flexible and abrasion resistant thanneoprene-coated ones. Moreover, because of better durability and compatibilityof silicones with nylon, a thinner coating is adequate. A silicone-coated air bagfabric made of 420 d/315 d nylon weighs only about 200 g/m2. They can,therefore, be packed in smaller modules [32].

For this reason, silicone coating is rapidly replacing neoprene coating fordriver-side air bags. Passenger-side air bags are made of uncoated nylon fabricsas the functional requirements are not that stringent. It is estimated that by theend of the decade, the requirement of coated air bag fabric will be between50–75 million sq. meters, making it one of the most important growth sectorsfor coated fabrics both in volume and in value terms [20].


A variety of fibers, both man-made and synthetic, are used as face fab-ric of carpets. The commonly used ones are cotton, wool, rayon, polyester,

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polypropylene, and nylon. The carpets are given a backcoating or a fabric back-ing to impart strength and durability. Three types of carpet backing are used inindustry [34,35].

(1) Secondary-backed carpet: in these carpets, a secondary backing fabric,usually jute or polypropylene, is bonded to the back of the carpet by anadhesive.

(2) Unitary coating: this consists of a simple application of an adhesive layeron the back of the carpet without any secondary backing.

(3) Foam backing: the back of the carpet consists of a thin cushion of foam asits integral part.

The backcoating process imparts certain important properties to the carpet,viz., tuft binding, dimensional stability, resistance to water, reduced pilling,resistance to edge fraying, etc. In order to achieve all these properties and toobtain adequate adhesion with the secondary backing, it is important to selectthe right adhesive.

Various materials have been used as adhesive over the years. They are naturalrubber latex, SBR latex, EVA emulsion, PVA emulsion, starches, etc. Out ofthese, SBR latex and carboxylated SBR latex are the most widely used adhe-sives. SBR latex is obtained by emulsion polymerization process. For back-coating purposes, the SBR latex is formulated with certain additives. Theyare calcium carbonate (extender), surfactant (frothing aids), and polyacrylatethickeners for adjusting the viscosity [36].

The tuft locking and stiffness depends on the coating weight of the latexas well as on the formulation such as filler content and styrene content of thelatex. The stiffness and tuft locking increases with styrene content and the addon. Increase in filler content decreases the tuft locking but increases stiffness[34].

Secondary-backed carpet is a type of carpet mainly used for residential pur-poses. Different methods are used for lamination of the backing jute fabric.These consist of applying an undercoat of SBR latex on the back side of thecarpet and an adhesive coat on the jute surface and bonding the two fabricsin between laminating rolls. The undercoat is usually frothed by air, for betterweight control, has higher viscosity (14,000–18,000 cps) and higher extendercontent (400–800 parts/100 parts latex). The adhesive coat has lower viscosity(9000–10,000 cps), lower extender content (300–400 parts/100 parts latex), andis unfrothed [35,36].

An important method of lamination is the pan coat, jute coat process, wherethe undercoat and adhesive coats are applied from pans to the carpet backand jute surface, respectively, by kiss-coating or roller-coating techniques. Thecoated surfaces are then bonded between press rolls and cured. In the direct-coating system which is more popular (Figure 7.8), the undercoat (frothed SBR

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Figure 7.8 Direct lamination of jute backing: (1) Carpet, (2) Frothed latex, (3) Bed plate,(4) Metering roll, (5) Jute fabric, (6) Adhesive, (7) Press rolls, and (8) Oven.

latex) is spread directly on the carpet back using a bed plate and a doctor roll;an adhesive coat is applied on jute surface by roller coat from pan, and the twosurfaces are bonded in the usual manner [34,35].

In the unitary backing process, a layer of adhesive, usually SBR latex, isapplied on the back side of the carpet by knife on roll, kiss coating, or rollercoating followed by curing. As mentioned earlier, no secondary backing is used.The latex used has lower extender content (100–150 parts/100 parts of latex)and lower viscosity (7000–9000 cps). The process gives better tuft lock anddimensional stability. These carpets are used where the traffic requirement ishigh [34,36].

For foam backing [37], a precoat of SBR latex is first applied on the backof the carpet and dried. A foam coat is next applied by blade or roller coating.The coated fabric is then dried and vulcanized in a stenter.

Different types of foam systems based on air-frothed SBR or SBR-NR latexblend are used. In the chemical gelation system, coagulation occurs due to desta-bilizing the colloid, forming a rubbery continuum. Two types of gelling agentsare commonly used: sodium silicofluoride, which operates at room temperaturewith delayed action, and ammonium salt-zinc oxide system, a heat-sensitive gel-lant. In a nongel foam process which is also quite popular, carboxylated latexis used along with water-soluble melamine-formaldehyde (MF) resin. Cross-linking occurs via the carboxyl groups of the latex with MF resin. The choiceof the foam system depends on the technical requirements, simplicity of theprocess, and cost.

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Textile fabrics are gradually replacing vinyls for car seats and interiorsbecause of their soft handle, design color, and pattern options. The fabricsused are generally pile fabrics, of nylon or polyester. The fabric is convertedinto a trilaminate, comprised of face fabric, a polyurethane foam of 2–10 mmthickness, and a lining fabric. The face fabric provides an attractive look, thefoam provides a soft cushioning effect, and the liner prevents the foam fromwear. The most common method of lamination is flame lamination. However,laminating with dry and hot-melt adhesives is emerging as an alternative. Thedifferent methods used for lamination are discussed below [38].


In this process, a roll of polyurethane foam passes over an open flame, result-ing in melting of the surface of the foam, which then functions as the adhesive.This material is then bonded to fabric, by passage through nip of the laminatingroll (Figure 7.9). The process is repeated for laminating the liner, on the backside of the foam.

The process is simple, does not require an oven, and gives fast line speeds. Theburning of the foam, however, liberates toxic gases such as hydrogen chlorideand cyanides, which have to be properly ventilated. The flame length has tobe properly adjusted as large flame burns too much of the foam, while a smallflame leads to insufficient melt and poor bonding. The process is restricted tofoams that can be melted.


Lamination can be done by scatter coating dry powder or by film adhe-sion of thermoplastic polymer. In scatter coating, powders of 20–200 µm of

Figure 7.9 Flame lamination.

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polyester, polyamides, or EVA obtained by cryogenic grinding of polymer gran-ules are scattered on the foam substrate. Foam containing the powder then passesthrough heaters to activate the adhesive by melting. The textile web is also heatedto near the softening temperature of the adhesive. The lamination of the foamand textile layers is carried out by passage through nip of the laminating rolls.In film adhesion, a film adhesive obtained in roll, by melt extrusion is placedon the foam substrate, and the two layers are passed through heaters to meltthe adhesive. Heated textile substrate is then laminated to the foam layer in alamination station. The laminates are cooled prior to winding. These methodshave the advantage in that no emissions are produced. They, however, requirelarge ovens and are unsuitable for temperature-sensitive fabrics.

Adhesives can be applied on the substrate for lamination by rotogravure or byspraying processes, but the viscosity and pot life of the adhesive are constraintsof their applicability.


Flocking is the application of short fiber (flock) on an adhesive-coated sub-strate in vertical position. The substrate may be woven or knitted textiles, leather,paper, polymer film, etc. Flocking is used for producing a variety of items withaesthetic appeal, such as draperies, bedsteads, carpets, and artificial fur andsuede. The flocking process provides an economical means of production ofpile texture.

The flock commonly used in trade are nylon or viscose, 1640 d, of 0.1 to0.6 mm length. For production of the flock, tows of fibers are extruded throughthe nip of rotating rolls and are cut by a blade. The speed of the rolls determinesthe flock length. The flock is then thoroughly washed, activated by cationicdetergent, and graded prior to electrostatic deposition on the substrate. Thetextile substrate used are mainly woven or knitted cotton/viscose fabric. Theadhesives are usually latices of NR, SBR, or aqueous dispersions of acrylicsor polyurethanes. The viscosity of the adhesive is adjusted using polyacrylatethickeners to prevent strike through of the adhesive during coating.

The flocking process consists of coating the textile substrate by knife on roll,rotary screen printing, or gravure roll coating for repeat patterns. Flock is thenapplied on the substrate electrostatically. For this purpose, the flock fibers areintroduced into a high voltage field as a result of which, as charge carriers, thefibers are transported to the adhesive-coated substrate at right angles. Adhesive-coated substrate forms the earth pole and is, in addition, vibrated by a set ofrollers. The fibers that penetrate the adhesive layer are retained there, forminga dense pile. Excess flock is removed by suction, and the product is passedthrough a tunnel drier, which is circulated by hot air at high temperature. Theproduct is cooled, excess flock is removed by brushing followed by suction,

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Figure 7.10 The flocking process: (1) fabric unwind, (2) coating of adhesive, (3) hopper containing,(4) rotating brush, (5) electrostatic depositer, (6) vibrating rolls, (7) suction, (8) drying oven, (9)cooling rolls, (10) brushing, (11) suction, and (12) winding.

and it is cut into suitable lengths and packed. A schematic layout of the processis given in Figure 7.10.


1. G. R. Lomax, Journal of Coated Fabrics, vol. 15, Oct., 1985, pp. 127–144.2. D. Morley, Journal of Coated Fabrics, vol. 14, July, 1984, pp. 46–52.3. Textiles, vol. 10, no. 3, 1981, pp. 65–68.4. Poly Vinyl Chloride, H. A. Sarvetnick, Van Nostrand Reinhold, New York, 1969.5. Polyurethane Handbook, G. Oertel, Hanser Publishers, Munich, 1985.6. B. L. Barden, Journal of Coated Fabrics, vol. 24, July, 1994, pp. 10–19.7. Kirk Othmer Encyclopedia of Chemical Technology, 3rd Ed., vol. 14, 1979,

pp. 231–249; 4th Ed., vol. 15, 1995, pp. 177–192, both John Wiley and Sons,New York.

8. Fibrous composite poromerics, W. Reiss, in Coated Fabric Technology, vol. 2,Technomic Publishing Co., Inc., Lancaster, PA, 1979, pp.45–56.

9. P. Durst, Journal of Coated Fabrics, vol. 13, Jan., 1984, pp. 175–183.10. J. Hemmerich, J. Fikkert and M. Berg, Journal of Coated Fabrics, vol. 22, April,

1993, pp. 268–278.11. B. Zorn, Journal of Coated Fabrics, vol. 13, Jan., 1984, pp. 166–174.12. R. Markle and W. Tackenberg, Journal of Coated Fabrics, vol. 13, April, 1984,

pp. 228–238.13. Man made leather, O. Fukushima, in Coated Fabric Technology, vol. 2, Technomic

Publishing Co., Inc., Lancaster, PA, 1979, pp. 119–131.14. C. Chu, Z. Mao and H. Yan, Journal of Coated Fabrics, vol. 24, April, 1995,

pp. 298–312.15. B. Kunst and S. Sourirajan, Journal of Applied Polymer Science, vol. 14, 1970,

pp. 1983–1996.16. H. Strathman, P. Schieble and R. W. Baker, Journal of Applied Polymer Science,

vol. 15, 1971, pp. 811–825.

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17. C. Lemoyne, C. Friedrich, J. L Halary, C. Noel and L. Monnerie, Journal of AppliedPolymer Science, vol. 25, 1981, pp. 1883–1913.

18. Absorptive vinyls, B. M. Murphy, in Coated Fabric Technology, vol. 2, TechnomicPublishing Co., Inc., Lancaster, PA, 1979, pp. 103–112.

19. J. Duncan, Journal of Coated Fabrics, vol. 13, Jan., 1984, pp. 161–165.20. Handbook of Industrial Textiles, S. Adanur, Ed., Technomic Publishing Co., Inc.,

Lancaster, PA, 1995.21. R. J. E. Cumberbirch, Textiles, vol. 16, no. 2, 1987, pp. 46–49.22. H. Mewes, Journal of Coated Fabrics, vol. 22, Jan., 1993, pp. 188–212.23. Architectural Fabrics, M. Dery, in Coating Technology Handbook, D. Satas, Ed.,

Marcel Dekker, NewYork, 1991.24. B. Foster, Journal of Coated Fabrics, vol. 15, July, 1985, pp. 25–39.25. G. R. Lomax, Journal of Coated Fabrics, vol. 15, Oct., 1985, pp. 127–144.26. Awning and Canopy Fabric Specifiers Guide, Industrial Fabric Association Inter-

national, 1996, 73/6, pp. 37–48.27. K. L. Floyd, Textiles, vol. 6, no. 3, 1977, pp. 78–83.28. G. R. Lomax, Journal of Coated Fabrics, vol. 15, Oct., 1985, pp. 127–144.29. E. Sowden, Journal of Coated Fabrics, vol. 13, April, 1984, pp. 250–257.30. L. H. Olson, NTIS report no. DAAG 53-76-C-0141, 1977.31. G. R. Lomax, Textiles, vol. 14, no. 2, 1985, pp. 47–56.32. E. T. Crouch, Journal of Coated Fabrics, vol. 23, Jan., 1994, pp. 202–219.33. F. A. Woodruff, Journal of Coated Fabrics, vol. 23, July, 1993, pp. 14–17.34. R. L. Scott, Journal of Coated Fabrics, vol. 19, July, 1989, pp. 35–52.35. K. Stamper, Journal of Coated Fabrics, vol. 25, April, 1996, pp. 257–267.36. Non apparel coating, D. C. Harris, in Coated and Laminated Fabrics New Processes

and Products, AATCC Symposium Proceedings, April 3–4, 1995, Denvers, MA,U.S.A., pp. 4–21.

37. Latex applications in carpets, D. Porter, in Polymer Latices and Their Applications,K. O. Calvert, Ed., Applied Science, London, 1982.

38. A comparative analysis of laminating automotive textile foam, J. Hopkins, in Coatedand Laminated Fabrics New Processes and Products, AATCC Symposium Proceed-ings, April 3–4, 1995, Denvers, MA, U.S.A., pp. 250–267.

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High-Tech Applications


AWARENESS of the risks and ill effects involved in working in an environ-ment full of pervasive liquids and chemicals has necessitated the use of

protective gear for employees in the workplace and persons in public places.In the developed world, the concerns related to the ill effects of toxic chemi-cals are much greater compared with developing and underdeveloped countries.Nevertheless, definite and distinct awakening is taking place, and new and strictlaws are being promulgated to protect individuals and the environment fromthe menace of toxic chemicals.

The toxicity of chemicals in general depends upon their structure, physi-ological action, and mode of exposure [1]. It may be safely stated that eachand every chemical known to date can be considered as toxic at some level ofintake. However, the most toxic chemicals developed and stockpiled for usein chemical warfare may be categorized in four classes, namely, blood agents,choking agents, vesicants, and nerve agents. Out of the above classes, vesi-cants and nerve agents manifest their effect through skin absorption. Vesicantsdamage body tisssue and form painful blisters that are difficult to heal. Nerveagents are absorbed and transported through the bloodstream where they blockthe enzyme acetyl choline esterase that plays a key role in the neurotransmis-sion cycle leading to incapacitation and fatality. Coated fabrics are used forprotection against these two classes of agents. Some of the important character-istics of common chemical warfare (CW) agents of concern are summarized inTable 8.1.

The toxicity and hazards associated with other common chemicals used inindustries are much below the chemicals listed in Table 8.1. Nevertheless, theworkers in plants and industries likely to be exposed due to the nature of thejob need to be protected from their ill effects.

2Contributed by V. S. Tripathi, DMSRDE, Kanpur, India.

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TABLE 8.1. Properties of Important Chemical Warfare Agents.

Volatility Time Taken forState at at 25◦C Appearance of

Name of Agent 25◦C (mg/M3) Effect Symptoms

1. Pinacolyl methyl phosphono Colorless 3060 Nerve Inhalation 1–5 min,fluoridate (SOMAN) liquid agent skin 30–60 min.

2. Isopropyl methyl phoshono Colorless 16400 -do- -do-fluoridate (SARIN) liquid

3. Ethyl N,N′-dimethyl Colorless 516 -do- -do-phosphoroamidecyanidate liquid(TABUN)

4. o-Ethyl-S-2-diisopropylamino Yellow liquid 16 -do- -do-ethyl methyl phosphonothioate(VX)

5. bis-(2-Chloroethyl) sulphide -do- 930 Vesicant 3 hrs.(S Mustard)

Coated fabrics play a key role in both civil and military applications as faras protection for the whole body is concerned. The whole gamut of coated fab-rics used for protection of the human body may be conveniently classified intwo broad categories, permeable or breathable and impermeable or nonbreath-able. As the names suggest, the former allows free ingress and egress of airfacilitating the dissipation of heat and evaporation of sweat, while the lattercompletely shields the wearer from the atmosphere. Obviously, the devicesmade of permeable-type fabrics can be used for longer duration of time dueto comparatively low heat stress. However, for many applications where largequantities of toxic chemicals are handled or liquid splash may occur completelydrenching the wearer, impermeable suits are preferred. Various types of per-meable and impermeable coated fabrics used for protection against differentchemicals and scenarios are discussed in this section.


Chemicals can enter the human system through inhalation, ingestion, or ab-sorption by the skin. The nature of the chemicals and the form and place ofexposure decides the type of protection required. Some prominent scenariosinclude during war where chemical warfare agents may be used; emergenciesinvolving accidental spills on highways; working with and handling hazardouswaste; laboratory work; radioactive contamination; manufacturing operationfor chemicals in pharmaceutical, electrical, and electronic industries; and inthe handling of pesticides, insecticides, and herbicides. The United States En-vironmental Protection Agency (U.S. EPA) has classified the exposure scenar-ios and level of protection required in four broad categories. Coated fabricsare used for protection against skin absorption. Obviously, the use of protec-tive clothing hampers the normal activities of the wearer, hence, a prudent

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TABLE 8.2. Categorization of Exposure Scenarios and Implements Requiredfor Protection.

ProtectionLevel Example of Scenario Implements Required for

A 1. Production, storage, and packaging Full face mask, suit completelyof extremely hazardous chemicals encapsulating the body, gloves

and overboots2. War field where toxic agents have

been used (damages due to inhalationand skin absorption) due todissemination of CW agents

3. Decontamination drillB. 1. Chemicals are highly toxic if Self-contained breathing

inhaled but not absorbed through apparatus-(SCBA) Skinthe skin protection suit not completely

2. Atmosphere with less than 19.5% sealedoxygen, e.g., fire, etc.

3. Splash of chemicals possibleC. 1. Industrial contaminants, type Full face mask with appropriate

and concentration known canister having high efficiency2. Particulate contaminants, e.g., particulate filter. Splash suit

radioactive dust preventing direct contact ofchemicals

D. Nuisance contaminants with Standard work clothingminimum hazards. No chemical and apronimmersion or splash

selection of chemical protective garments based on the hazard and risk ofexposure anticipated is very important. Table 8.2 gives an idea of possiblescenarios and the recommended protective equipment required in the givenscenarios.

It may be noted from Table 8.2 that the stringent requirements of level Ahazards require complete encapsulation. This drastically reduces the time ofuse for chemical protective clothing (CPC) made of impermeable material. Incivilian applications, it is possible to rotate the deployed manpower at shortintervals to finish the task. However, in military applications, especially in warfield rotation, it is not possible, hence, permeable/breathable suits are preferred.CPC requirements stipulated in B, C, and D levels of hazards are not critical,nevertheless, they should provide protection against chemical splashes in B andC levels of hazards.


Impermeable chemical protection clothing is normally made either by bar-rier coating on a fabric or films as such. The fabric used for coating or as a

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carrier of the barrier compound provides necessary strength to the clothing.Fabrics made of polyester, polyamide, and cotton and their blends are com-monly used as carrier fabrics. In disposable limited-use garments, nonwovenfabric laminated with a barrier film is preferred. The materials used for civil andmilitary applications (against CW agents) are different, and they are discussedbelow. Civil Applications

Since the first use of chemicals for military purpose in World War I, tremen-dous advancement has taken place in understanding the barrier properties ofpolymeric materials vis-a-vis different types of chemicals in liquid or vaporforms. In the beginning of the 20th century, natural rubber was the only ma-terial available for coating. Prior to World War II, synthetic elastomers suchas neoprene, polyvinyl chloride, and butyl rubber replaced natural rubber incivil and military CPCs. With the development of newer polymers like flu-oropolymers, CPCs using a wide spectrum of elastomers, and thermoplas-tics were developed by leading manufacturers all over the world. Coextrudedpolymeric films have become a material of choice in limited-use disposableclothing. A list of different materials used in impermeable CPCs is given inTable 8.3 [2].

The criteria for selection of a material for a civil application depend onthe permeation of the toxic vapor and penetration of the challenge liquid asevaluated by ASTM 739 and 903.

TABLE 8.3. Common Barrier Materials in Impermeable CPCs.

Elastomers—Unsupported/Reinforced• butyl/bromobutyl• chlorobutyl• fluoroelastomers (Viton)• urethane

Plastic Film Laminates—Coating• chlorinated polyethylene• PTFE• polyethylene• polyvinyl chloride• polyvinylidene chloride

Bicomponent Constructions• fluoroelastomer/butyl• fluoroelastomer/neoprene• PVDC/polyethylene• neoprene/PVC

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For military applications, the barrier properties are evaluated using S Mustardas the probe. This is considered the most penetrating of all the CW agents. Amaterial is considered suitable for the protection of inanimate objects if it doesnot absorb more than 50% of the placed 1 µL of S Mustard on 1 cm2 area of thematerial in 6 hrs. In personnel protection, vapor penetration should be very lowbecause exposure to even 75 µg/m3 concentration for one hour is enough toproduce blisters. The test procedure of these materials is discussed in Chapter 9.

Two types of materials are used for fabrication of impermeable suits, etc.:(a) multilayer sandwich type and (b) coated nylon or polyester fabric [3]. Inthe former type, a barrier film of material such as polyvinylidene chloride,polyamide, or polyester is sandwiched between weldable polyolefin films. Sand-wich layers are biaxially oriented before or after lamination for better mechan-ical strength. The overall thicknesses and weights of such multilayer films are100 µms and 100–150 g/m2, respectively. For heavy-duty application, such asadhoc collective protection, decontamination, and disposal of munitions, coatedfabrics having much higher strength than multilayer films are preferred. Butylor perfluorocarbon rubber is used for coating, and the weight of the fabric variesbetween 250–500 g/m2.


Permeable fabrics allow free passage of air, permitting sweat of the wearerto freely pass out as water vapor. The advantage gained in suits made of suchfabrics is significant as the physiological load in the form of heat stress is muchless compared to that of the impermeable suits. It is possible to indulge inlight to medium work schedules wearing these suits for a sufficient length oftime.

A layer of high surface area microporous carbon (pore width <20 Å) isimpregnated/coated on different carriers for attenuation of challenge concen-tration of the chemical agents. The role of carbon in a breathable fabric is verycritical, in that, it should preferentially adsorb chemical agents with minimaldesorption during usage. High surface area microporous carbon performs thistask remarkably well. Desorption from micropores where adsorption forces areenhanced due to proximity of walls in slit-shaped pores is comparatively dif-ficult. Fortunately, such active carbons are very good adsorbents of chemicalagents that are adsorbed through the skin at ambient temperature. Even a bed of0.5–1 mm of microporous carbon available in NBC fabrics provides very goodprotection, provided the rate of adsorption is high. Different permeable fabricsystems in use globally are described below along with their salient features[3,4].

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Lightweight, low density, thin (1–2 mm) polyurethane appears to be goodcandidate matrix for impregnation of carbon. In fact, comparatively large quan-tities of carbon (200 g/m2) can be impregnated in foam material, yet loss ofcarbon during work schedules and military operations is insignificant. How-ever, the insulation properties of foam and its ability to adsorb fluids are seriousdisadavantages that cause a lot of physiological stress. Moreover, being vo-luminous material, the suits made of the carbon-impregnated foam cannot bepacked in a small space. The deterioration of foam, especially in a hot andhumid atmosphere, is another drawback. Suits of this type are currently beingmanufactured and used in France and some other countries. Carbon-Impregnated Cotton Flannel and Nonwoven Fabric

Activated carbon in very fine powder form can be coated on open-structurecarriers such as cotton flannel and nonwoven fabric using a suitable binder.In the case of flannel, air permeability is on the low side. This type of carbon-coated fabric is used in China for NBC suits. Thin wadding of nylon or polyesterreinforced by open-structure cotton scrim is probably the best carrier for ac-tive carbon, and this type of coated fabric gives very good air permeability.Polychloroprene is used as a binder because of its flame retardant properties.It reduces the adsorbability of carbon only marginally. The carbon content perunit area (45–80 g/m2) is about one-third compared to foam-impregnated ma-terial, nevertheless, a faster rate of adsorption due to the finer particle size andbetter air permeability more than compensates this shortcoming. Such suits aremade and used in the U.K. Bonded Spherical Carbon Adsorbents

In this system, microspheres of activated carbon having diameter 0.5–1.0 mmare point bonded on a carrier fabric. Approximately 150 g/m2 carbon loading isachieved in this way, and this gives much better life when compared to carbon-coated fabrics. However, due to the larger granule size, the rate of adsorptionis low, and larger quantities of carbon per unit area result in more weight,higher heat stress, and more cost. This type of microsphere-coated fabric ismanufactured in Germany and the U.S. for military applications. Active Carbon Fabric (Charcoal Cloth)

Activated surface area microporous adsorbent media in a fabric form wasdeveloped by Maggs [5] using viscose rayon fabric as precursor. The manu-facturing process of activated carbon fabric comprises of pretreatment of theprecursor fabric with a Lewis acid solution and the carbonization and activation

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in a carbon dioxide atmosphere. The surface properties of the end product canbe controlled by careful selection of operating parameters [6]. This type of ad-sorbent media can be used in protective clothing after proper lamination withwoven or nonwoven fabric. The quantity of activated carbon available per unitarea of fabric is quite high (100–120 g/m2), and the rate of adsorption is alsovery high. In spite of all the advantages, poor mechanical strength of charcoalcloth has found limited use in chemical protective clothing.


A number of high technology fibers have been deveoped in recent years, someof them are based on microencapsulation technology [7,8]. These include fab-rics that release perfume on rupture of the microcapsule (fragrance fabrics) andfabrics that change color with temperature. An example of the color-changingfabrics is Toray’s Sway brand of skiwear. This is PU-coated nylon that containsmicrocapsules containing heat-activated dye. A ski suit can be made to changecolor from bright red at the slopes outside and white indoors by the side ofa fire. The color-changing fabric is of great interest because it has potentialapplication for camouflage. The dyes used in these fabrics are thermochromicin nature which change color with temperature.

Numerous inorganic and organic compounds show thermochromism, and thesubject has been reviewed [9]. Inorganic compounds show both reversible andirreversible thermochromism due to phase change or due to change in ligandgeometry of the metal complexes. Temperature-indicating paints showing irre-versible thermochromism has been used for a long time as a warning of hot spotsand as a record of heat history in the electrical and chemical industries. The inor-ganic compounds have not found favor in textile applications as the color changegenerally occurs in solution or at high temperatures. The ideal thermochromicsystem for apparel application should show reversible thermochromism be-tween ambient and body temperature. Reversible thermochromism in the solidstate is exhibited by many organic compounds. These compounds undergostereoisomerism, molecular rearrangements, or are liquid crystals. Among theliquid crystals, the most important systems are cholesteric mesophase. Thesehave limited use in textiles due to high cost, color restricted to deep shades,low moisture resistance, and low color density. Sterically hindered ethylenecompounds such as bianthrone and dixanthylene also show thermochromism.These compounds are characterized by at least one ethylene group, a num-ber of aromatic rings, and a hetero atom, usually N or O. The ethylenic bondprovides a route for extension of conjugation and places restrictions on pos-sible molecular orientation. As the temperature is increased, the moleculechanges to a different stereoisomer, which is colored. These compounds showtransition above their melting point ∼150◦C, thus are not suitable for textileapplications.

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Figure 8.1 Tautomers of crystal violet lactone.

Certain dyes undergo keto-enol type of tautomerism. Such tautomeric rear-rangements can lead to an increase in the conjugation and formation of a newchromophore, leading to color development. Such rearrangements can be in-duced by a change of temperature, pH, or polarity of the solvent, resulting inthermochromism. These dyes are extensively used for textile applications. Themost common types are fluorans, crystal violet lactones, and spiro pyrans. Allof these dyes undergo ring opening rearrangements [10]. The equilibrium ofcrystal violet lactone is shown in Figure 8.1.

Reversible thermochromism composition is made from such dyes, along witha color developer containing acidic protons capable of proton transfer or strongH bonding to the dye molecule. The dye and the developer are dissolved in anonvolatile solvent, and the ternary composition is encapsulated. On heating,the organic solvent melts, resulting in color change. Some commonly useddevelopers are bisphenol A, bisphenol B, 1,2,3-triazoles, thioureas, etc. Variouscompounds have been used as solvents, but the most common are aliphaticalcohols like stearyl alcohol. A puzzling feature of this system is that they arecolored at low temperature and turn colorless at high temperature. Differentreasons have been attributed to explain the same [10].

It has already been mentioned that the color development of the ternarysystem depends on the melting point of the solvent used. In order to ensurea homogeneous mixture throughout the color development stage, it is neces-sary to keep it in a closed system by microencapsulation. In a microcapsule,which is a small solid particle of 1–1000 µm size, there is a core contain-ing the thermochromic system and a coating or shell of a polymeric material.Two processes are prevalent in the literature for microencapsulation of ther-mochromic dye systems. They are complex coacervation and interfacial/in situpolymerization.

In complex coacervation, two polyelectrolytes of opposite charges are used,such as gelatin and gum arabic [11]. At a pH of <4.7, the gelatin is cationic,and gum arabic is anionic. The core material is initially dispersed in the gelatinsolution. To this dispersion, a solution of gum arabic is added, and the pHis adjusted to ∼4.0. This causes a liquid complex coacervate (droplets) ofgelatin-gum arabic and water to form, which surrounds the core to form embryocapsules. The system is then cooled to gel the shell. The shell is then cross-linkedwith glutaraldehyde and dried to form a free-flowing powder of microcapsules.

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The shell formation in interfacial polymerization occurs due to polyconden-sation at the surface of the core material. Core material and one of the reactants,like multifunctional acid chloride or isocyanate, are mixed together to form awater-immiscible mixture. This mixture is dispersed in water with the aid of anemulsifier. The other reactant, e.g., multifunctional amine or alcohol, is addedto the aqueous phase. Interfacial polymerization occurs at the surface of thecore, forming the shell of the microcapsule. In situ polymerization is the tech-nique adopted for forming shells of aminoplasts. The solid core material isfirst dispersed in water that contains urea, melamine, or water-soluble urea-formaldehyde condensate. An anionic polymer is added to enhance aminoplastshell formation. Formalin is then added, and the pH is adjusted to 2–4.5. Onheating (40–60◦C), shell formation occurs.

The microcapsules are used as conventional pigments and are coated on fiberor fabric with the aid of polymeric binders.

Shibahashi et al. have described the above technology in coating a varietyof fibers [12]. The fibers have been converted into yarns, nonwoven fabrics,and knitted and woven fabrics of various constructions. All of these show ther-mochromic effect. By proper selection of the dye system, they have been ableto obtain thermochromic effects at temperatures ranging from −30 to 100◦C.For uniform color change, proper pigment particle size has been selected de-pending upon the density and the denier of the fiber. The proportion of dye,developer, and solvent are critical for optimum results. Besides, the add onhas to be carefully chosen to keep a balance between clear color change andthe texture of the textile material. In a typical formulation, a thermochromiccomposition consisting of 1 part by weight of crystal violet lactone, 3 partsof benzoyl-4-hydroxy benzoate, and 25 parts of stearyl alcohol, was encapsu-lated by coacervation in gelatin-gum arabic. The microcapsules were coated onthe fibers by dipping in a polyurethane emulsion. The resulting fiber exhibitedreversible thermochromism, turning blue above 53◦C and becoming colorlessbelow that temperature. By coating the system on a dyed fabric, they have beenable to achieve change of color from one colored state to the other. Differentbinders have been used for coating. They include low melting point thermo-plastics, natural and synthetic resins, and emulsions. Coating has been donemainly by dipping or spraying. The applications include apparel, toys, artifi-cial flowers, etc. Thermochromic patterns have been obtained on fabric usingthermochromic and uncoated fibers.


Textile materials are being increasingly used for architectural purposes. How-ever, these lack the thermal insulation of the conventional building materials.The thermal insulation can be increased by a double-shelled construction orby adding a layer of foam to the textile material. A new way to improve the

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thermal insulation can be the application of phase change materials (PCM).When a substrate containing PCM is heated, by solar radiation, the increasein temperature of the substrate is interrupted at the melting point of the phasechange material, due to absorption as latent heat. The temperature will rise onlywhen all the solid has melted. Conversely, during the cooling process at lowambient, the drop in temperature is interrupted at the solidification temperature.The heat flux through a material containing PCM is thus delayed in both heatingas well as cooling, during the process of phase change. This thermal insulationeffect is dependent on temperature and time; and being temporary in nature, itcan be termed dynamic thermal insulation.

Vigo and Frost [13,14] have incorporated polyethylene glycol of differentmolecular weights as PCM in hollow fibers resulting in a 2–2.5 times increase inheat content compared to the untreated material. Similar results were obtainedby treating textile materials with aqueous solution of the PCM by pad drymethod. The main drawback of the process is that the PCMs are water soluble.

Considerable improvement in technique has been done by Pause [15], whohas used hydrophobic higher hydrocarbons like dodecane, octadecane, etc., asPCMs. These compounds were encapsulated to form microcapsules of 1–60 µmsize. The microencapsulated PCM was applied by a thin layer of lacquer onPVC-coated polyester with foam backing. In order to meet the requirementsof widely varying ambient of winter and summer months, two PCMs havingdifferent transition temperatures were used. A procedure was devised to mea-sure the dynamic thermal insulation properties. A comparative study of coatedfabric containing 40 g/m2 micro-PCM showed a fivefold increase in thermalinsulation. The technology has great potential and further development work isin progress.


Camouflage nets are meant to conceal military equipments and objects fromdetection and attack by an enemy. It has a great effect also on the morale ofthe fighting forces. Historically, camouflage nets were first used during WorldWar I. The earlier nets were made of hemp/cotton twine, garnished with jute stripscrims, and dyed/coated with green and brown colors. These nets blended in withthe surroundings and prevented detection by naked eye or by binoculars. Thesenets had serious limitations, such as poor camouflage properties, prematurefading of colors, susceptibility to fungus growth, short life, and high waterabsorption, resulting in a great increase in weight of the net when wet.

With the rapid developments of sophisticated surveillance systems such as ac-tive and passive infrared sensors, infrared line scanners (IRLS), forward lookingairborne radar (FLAR), side looking airborne radar (SLAR), millimeter waveradar, etc., it became imperative to develop camouflage nets that would protect

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the objects from detection by various sensors. The requirements of camouflagenet can be summarized as follows:

� small volume and light weight� strong and durable� waterproof and fire resistant� colors and patterns similar to the surroundings� easy to deploy, transport, and handle� conceals objects against detection by sensors

To meet these requirements, modern camouflage nets are wholly made ofsynthetic textiles. The nets available offer different levels of protection, viz,against visual and near infrared; visual, near infrared and microwave; visual,near infrared, thermal infrared and microwave; and ultraviolet for snow terrains.


These nets camouflage the objects in the visual (400–700 nm) and near IR(700–1200 nm): regions of the electromagnetic spectrum. That is, they offerprotection against visual detection and against NIR sensors, such as night visiondevices and image intensifiers. In order to blend the objects with the surround-ings, the reflectance of the nets in the visible and near IR regions (400–1200 nm)should match that of the surroundings. To achieve these characteristics in thenet, it is necessary to know the reflectance pattern of the objects constitutingthe surroundings in this spectral region. A typical spectrum of green vegetationcontaining chlorophyll is given in Figure 8.2, showing an IR reflectance valueof about 50%. From such spectra, the IR reflectance of other inanimate objectsof the surrounding area are obtained (see Table 8.4) [16,17].

Figure 8.2 Spectrum of green vegetation in visual and near IR.

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TABLE 8.4. IR Reflectance Values of Some Common Objects.

Constituents Reflectance %

1. Green vegetation 50–702. Concrete 40–503. Damp soil 10–154. Dry soil 15–205. Sand 30–406. Building bricks 30–407. Galvanized iron 15–20

Two types of terrains are generally considered for camouflage, green vege-tation and desert region. The color and IR reflectance of the net should matchthose of the terrain. Different nets are available for different terrains. A singlenet with reversible color pattern is also available.

The camouflage net consists essentially of two components, a netting formingthe base and a garnishing material, usually coated fabric, that is fixed to thenetting with clips. The netting is a square mesh of nylon twine, mesh sizevarying from 50–80 mm with soft vinyl coating or a flame retardant treatment.The garnishing material is usually PVC-coated nylon fabric incised in a suitablepattern. A lightweight fabric is taken as the base fabric (∼70 g/m2) that enhancesthe strength of the garnishing material. Unreinforced PVC films are also used forgarnishing. The color scheme of the garnishing material depends on the terrainof deployment [18]. The colors for green belt are olive green, deep brunswickgreen, and dark brown. For the desert region, the colors are light stone, darkstone, and dark brown.

For effective camouflage in visual and near IR regions, the garnishing materialshould not only be of the desired color but should also have IR reflectance similarto that of the surroundings. For this purpose, pigment composition for PVC isfirst selected to obtain a visible match of the color. The level of IR reflectanceis adjusted to the required value by introducing in the formulation normallya small percentage of high reflecting (e.g., TiO2) or absorbing (e.g., carbonblack) pigments. The nets are available in unit sizes of 1–1.7 m, which can beattached together to form a net of required size and shape. The incision in thegarnishing replicates the shadows that occur in nature and breaks up the outlineof an essentially rectangular or tubular nature of a vehicle or weapon system.The weight of the nets varies from 200–350 g/m2.


The most important component of a radar scattering net is the base fabricof the garnishing material. It consists of a specially designed nylon fabric in

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which is incorporated metallized yarns or aluminized polyester threads at reg-ular intervals, both in warp and weft directions. The metallized threads forma regularly spaced grid structure in the fabric. The fabric is then coated withPVC compound containing pigments to meet the visual and near IR reflectancerequirements. The coated fabric is incised in a definite pattern. By properlydesigning the grid structure of the metallized thread and with proper incisionof the garnishing material, it is possible to scatter the incident microwave radi-ation from the radar, in a manner similar to that of the surroundings, leading toconcealment of the object from radar. Nets made by Barracuda of Sweden havean attenuation of 10 dB against 3 cm radar (X band). It is claimed that the echoobtained from the object plus the net corresponds approximately to that of thesurroundings [19].


Snow has high reflectivity in UV region ∼90%. A standard white shade, onthe other hand, has a poor UV reflectance, ∼10%. Thus, a military object whencovered by standard white shade fabric in snow terrains, shows up as a blackpatch in a white background when viewed by reconnaisance devices using a UVfilter at 350 nm. Camouflage nets for snow regions have a garnishing materialof white PVC-coated nylon fabric having a UV reflectance of 75% minimumat 350 nm providing camouflage in both visual and UV regions.


The incorporation of metal into textiles dates back to the Roman era, whenthey were mainly used for decorative purposes. The tinsel yarns used to addglitter to fabrics were made by flattening thin wire or sheets of noble metallike gold or silver. By the 1930s, aluminium foil strips coated on both sidesby cellulose acetate-butyrate, to prevent them from tarnishing, were used. Theyarn could be colored by anodizing. All of these yarns had poor compatibilitywith the more flexible and extensible textile yarns [20]. After the developmentof vapor-deposited aluminized polyester in the 1960s, 1 mm wide strips of thesefilms were used as yarns, with much improved flexibility.

With the advancement of technology, metal/conductive textiles found exten-sive functional applications. These materials have high electrical conductivityand radar reflecting property, yet are lightweight and flexible. Various methodshave been developed to coat fibers and textile materials by metals, and theseare as follows [21]:

� coating metal powder with binders� vacuum deposition

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� sputter coating� electroless coating


a. Metal coating with a binder: the process is similar to conventional poly-mer coating. High leafing aluminium pastes (65–70%) are incorporated intoa polymeric carrier, like synthetic rubber, PVC, polyurethanes, silicones,acrylic emulsions, etc., and spread coated on the fabric. The coating methodmay be conventional knife or roller coating. The adhesion, flex, and chemicalresistance of the coated fabric depend on the type of polymer used, but theyare not highly reflective.

b. Vacuum deposition: in this process, the substrate to be coated is placed in achamber over a set of crucibles containing the metal to be coated in the formof a powder/wire. The chamber containing the whole assembly is evacuatedto 0.5–1 torr. The crucible is heated by resistance heating to melt the metal.The temperature of heating is so adjusted that the vapor pressure of themetal exceeds that of the chamber pressure, so that substantial evaporationof the metal takes place. The temperature required for aluminium is about1200◦C. The roll of web to be coated is passed over a cooled drum placedover the crucibles. The metal atoms coming out of the molten metal hit thesurface of the web to be coated and condense in the form of solid metal as itpasses over the crucible. The production speed is quite high, ranging from150–500 m/min. The items to be coated should be pretreated for properadhesion of the metal. Continuous metal film coatings can be formed on justabout any surface, film, fiber, or fabric with thicknesses ranging from micronto millimeter. Several metals can be vacuum evaporated, most common beingaluminium, copper, silver, and gold. Difficulty arises in the case of metals,which sublime rather than melt and boil [21,22].

c. Sputter coating: the equipment consists of a vacuum chamber containing aninert gas, usually argon, at 10−3 to 10−1 torr (Figure 8.3). The chamber isequipped with a cathode (target), which is the source of the coating material,and an anode, which acts as a substrate holder. Application of an electricalpotential of the order of 1000 Vdc, between the two electrodes, produces aglow discharge. A flow of current occurs due to movement of electrons fromcathode to anode. The electrons ionize the argon gas. The argon ions areaccelerated toward the cathode at a high speed due to high electric potential.The bombardment of the energetic ion on the target results in a transferof momentum. If the kinetic energy of the striking ion is higher than thebinding energy of the surface atoms of the material of the target, atoms aredislodged or sputtered from its surface by a cascade of collisions. Typically,the threshold kinetic energy of the ions should be between 10–30 ev for

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Figure 8.3 Sputtered coating process.

sputtering from the surface. Some of the ions striking the target surfacegenerate secondary electrons. These secondary electrons produce additionalions, and the discharge is sustained. Considerable heat is generated during thesputtering process, and it is necessary to cool the target. The sputtered atomsand ions condense on the substrate to form a thin film of coating [23,24].The relative rates of deposition depend on sputter yield, which is the numberof atoms ejected per incident ion. The sputtering yield varies with the targetmaterial and increases with the energy of the incident ion. The method isapplicable to a wide range of materials and gives more uniform coating withbetter adhesion than simple vapor deposition. The process is, however, moreexpensive, and the rate of deposition is lower (∼30 m/min).

d. Electroless plating: it is a process to deposit metal film on a surface, withoutthe use of electrical energy. Unlike electroplating where externally suppliedelectrons act as reducing agent, in electroless plating, metallic coatings areformed as a result of chemical reaction between a reducing agent and metalions present in solution. In order to localize the metal deposition on a par-ticular surface, rather than in the bulk of the solution, it is necessary thatthe surface should act as a catalyst. The activation energy of the catalyticroute is lower than the homogeneous reaction in solution. If the depositedmetal acts as a catalyst, autocatalysis occurs, and a smooth deposition isobtained [25,26]. Such an autocatalytic process is the basis of electrolesscoatings. Compared to electroplating, electroless coating has the followingadvantages:

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(1) Nonconducting materials can be metallized.

(2) The coating is uniform.

(3) The process is simple and does not require electrical energy.

Electroless coating is, however, more expensive.

For successful deposition of coatings, only autocatalytic reduction reactionscan be used. As such, the number of metals that can be coated are not many.Some of the common reducing agents are sodium hypophosphite, formalde-hyde, hydrazine, and organoboron compounds. Each combination of metal andreducing agent requires a specfic pH range and bath formulation. The coatingthickness varies between 0.01 µm to 1 mm.

A typical plating solution consists of

a. Metal salt

b. Reducing agent

c. Complexing agents, required in alkaline pH and also to enhance the auto-catalytic process

d. Buffers

e. Stabilizers, which retard the reaction in the bulk and promote autocatalyticprocess

Some important metal coatings are discussed below.

a. Copper: the most suitable reducing agent is formaldehyde. The autocatalyticreaction proceeds in alkaline pH (11–14). The commonly used complexingagents are EDTA, tartarate, etc. The overall reaction is given by

Cu+2 + 2HCHO + 4OH− → Cu + 2HCOO− + H2 + 2H2O

b. Nickel: sodium hypophosphite is the most popular reducing agent for nickel.The autocatalytic reaction occurs in both acidic and alkaline pH. Sodiumcitrate is used as buffer and complexing agent. The reaction is given as

Ni+2 + 2H2PO−2 + 2H2O → Ni + 2H2PO−

3 + H2 + 2H+

The coating obtained by sodium phosphite also contains phosphorus(2–15%).

c. Silver: the plating solution consists of ammoniacal silver nitrate with formal-dehyde, hydrazine, and glucose as reducing agents. Because the autocatalyticactivity of silver is low, thick deposits cannot be obtained.

Nonconducting materials like polymers are given an etching treatment bychromic acid, followed by a catalytic treatment using stannous chloride or

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palladium chloride solution. Electroless plating of textiles is being adopted fordifferent functional applications. The details of which are mostly covered bypatents.


Metallized fabrics and fibers find diverse applications, many of them in high-tech areas. Some of the important uses are described below. Protective Clothing

The heat reflecting property of the metallized fabrics is used for protectionagainst intense radiant heat for short duration [20,27,28]. Such suits are requiredby firemen during firefighting and workers in the steel industry, for protectionagainst blast furnace radiaton and molten metal splash. The development ofthese suits has been stimulated by the widespread withdrawal of asbestos asa heat-resistant material. Three types of suits are in use by firemen that offerdifferent levels of protection given by the thermal protection index (TPI). TPIradiation and flames are defined as the time in seconds for the temperature ofthe back surface of the clothing assembly to rise by 25◦C above the ambientwhen exposed to a standard radiant heat source of 20 kW/M2 at a distance of200 mm or exposed to a standard heat source of burning hexane (BS 3791). Allof these suits contain a heat-reflective fabric, which consists of a polyester filmvapor deposited by aluminium to a thickness of about 200 Å on both sides andis laminated to glass fabric by a high temperature adhesive. The smooth surfaceof the polyester provides a high level of reflectivity.

a. Approach suit: this suit is meant for close approach to fires and protectionagainst radiant heat only. It is made of different layers, the outermost beingthe heat-reflective fabric, followed by a neoprene-coated fabric as a moisturebarrier, and preferably an inner layer of flame retardant cotton fabric, incontact with the body. It has a TPI of 50 against radiation (BS 3791).

b. Proximity suit: these suits are for operating in proximity to flame and offerprotection from radiant heat and occasional flame lick. The suit consists ofat least three layers: the outer shell made of heat-reflective fabric, a moisturebarrier, and a thermal barrier (NFPA 1976). The insulation layer may containKevlar® or carbon fiber fleece. The TPI is 80 for radiation and 18 for flame.

c. Entry suit: the suit permits firemen to enter the flame for a short period forrescue operation. The suit contains several layers of heat-reflective fabricand insulating layers, but the outermost layer should be noncombustible,like asbestos or special glass fabric with conductive coating. The TPI is thehighest for these suits—300 for radiation and 100 for flames.

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Kiln entry suits for workers also contain an aluminized polyester heat-reflective layer. Radar Responsive Fabrics

Metallized fabrics are capable of reflecting electromagnetic radiation and canact as a radar target by giving strong echo. They have advantages over metalsincluding being lightweight and easier to fabricate into different objects [29].One of the main applications of these fabrics is for making lifesaving devices forlocating persons marooned in high seas. These include caps for lost fishermen,life jackets for aircraft crew forced to drop into the sea during an emergency,and foldable radar fabric reflectors for life rafts. Besides, movements of me-teorological balloons are tracked by providing targets of metallized fabrics onthem. In defense application, a target banner is towed behind an aircraft at adistance for practicing surface-to-air firing by soldiers after locating the sameon a radar. The fabric for the banner is made of monofilament yarn (polyethy-lene, nylon, viscose) containing metallized threads of duralumin or silver, inboth warp and weft directions, in plain-weave construction. Target parachutesand target sleeves are also used for similar firing practice. Static Electricity Control

Rubbing action between two nonconducting materials tends to generate astatic electrical charge. Some typical examples are walking on carpet, flow ofhydrocarbon gas through plastic pipe, and reciprocal motion between textiles.The charge buildup may suddenly release in the form of a spark. This maycause fire or explosion in contact with flammable substances. Static electricitymay also cause damage to electronic circuitry. The conductivity of commontextile fibers is of the order of 10−13 (ohm · cm)−1. Increasing the conductiv-ity to 10−3 to 10−10 (ohm · cm)−1 range is usually adequate for dissipationof static charge [30]. Some examples of antistatic fabrics are staff apparel inelectronic industry, filter panels, conveyor belt reinforcement, antistatic floor-ing, etc. A common method to reduce static charge buildup is to impart ahygroscopic finish to the material by phosphoric acid ester, quaternary am-monium compounds, etc. These compounds absorb moisture from the atmo-sphere, increasing the conductivity to ∼10−10 (ohm · cm)−1 for static control.The drawbacks of these compounds are that their effectiveness depends on thehumidity of the environment and that they are removed by washing [20,30].Incorporation of conducting fiber is a sure way of dissipating static charge intextile. These fibers are carbon fibers, metal fibers, and metallized fibers. Car-bon fibers impart black color to the textile, while metal fibers are difficult to mixwith other fibers due to their brittleness and high density. Metallized fibers ob-tained by vacuum deposition or electroless coating have the advantage of being

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processed like any other textile fiber. Development of copper-, nickel- and silver-coated fibers by chemical method has been reported for antistatic application[30–32]. Electromagnetic Interference Shielding

Any electrical or electronic device, including household appliances, gen-erates electromagnetic radiation causing interference. These, in turn, can bedisturbed by other devices. Various other sources of interference are cosmicrays, lightning, and high voltage power cables. The trend toward faster, morepowerful electronic equipment and denser circuitry has increased the possibilityof electromagnetic and radio frequency interference (EMI and RFI). This de-velopment presents a challenge to scientists to control EMI emission as well asto shield sensitive electronics from EMI to meet strict international regulations.Metallized fabrics are emerging as a material of choice for EMI/RFI shielding ofsensitive equipment, particularly in defense and in aerospace. The conductingfabric can be tailored to ready-to-use adhesive tapes, curtains, bags, etc.

In order to understand the shielding process, let us consider an electro-magnetic wave impinging on a shielding screen. The incident wave will un-dergo reflection from the surface, absorption by the material of the screen, andsecondary reflection. The attenuated wave is transmitted from the other side ofthe screen. The shielding efficiency is expressed in decibel (dB) and is givenby:

SEdB = 20 log10 EI /ET (for electrical field)

SEdB = 20 log10 HI /HT (for magnetic field)

where EI and HI are the incident electrical and magnetic fields, and ET andHT are the transmitted fields. The shielding efficiency increases with the con-ductivity of the fabric and thickness of the coating.

Various metal-coated fibers and fabrics have been reported in the literature forEMI shielding. Texmet brand of fibers are obtained by deposition of copper andnickel on acrylic fiber by a chemical process. A coating thickness of 0.3 µmgives a conductivity of 103 (ohm · cm)−1. The fiber is available in the formof crimped staple fiber or as continuous tows. Nonwoven fabrics of differentconductivities have been made by incorporating Texmet fiber into other textilefibers, like polyester, polypropylene, and acrylic. Nonwoven with 70% Texmetloading showed very good microwave reflection and a transmission loss of over65 dB in the 8–12 GHz range [30]. Temmerman [31] has described an electrolesscoating process known as Flectron (a brand name of Monsanto Chemical Co.).Metal coating of copper, nickel, silver, etc., singly or a combination of onemetal with an overcoat of a second metal has been done by this process on

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a variety of substrates, including woven, knitted, nonwoven fabrics, choppedstrands, and films. The metal content of these fabrics ranges from 14–24 g/m2,with surface resistivity varying from 0.04–(0.43 ohms/sq (see AATCC testmethod 76-1995). A coating of 15 g/m2 metal, Cu, Cu/Ni, or Sn/Cu on nylonnonwoven gave a shielding between 56–90 dB in the frequency range of 100to 10,000 MHz. It was estimated that these metallized fabrics provide 95–97%of the shielding than would be provided by an equivalent mass of metal in foilform. The metal layers are well adhered on the substrate. Silver-coated nylonfiber and nylon fabric have also been developed by Statex system of coating fordifferent applications [32], including EMI shielding. Shielding of over 40 dBhas been obtained in 90 GHz to 500 MHz range using silver-coated nylonfabric.

Apart from the above specialized applications of metal-coated fibers/fabrics,they are also used for several other purposes. The passage of an electric currentthrough metal-coated panels of fabric results in resistive heating. This propertycan be used for making heated garments, gloves, blankets, and as an IR tankdecoy [33]. Some other uses are ironing board covers and pleated windowshades for thermal protection. A comprehensive list of applications of thesefabrics has been given by Smith [21].


Several people have been working on the development of conducting polymer-coated textiles. Major problems of these coatings are their poor environmentalstability and difficulty in processing them from solution or melt. As such, thetechnology is still in the developmental stage. The polymers that have beentried are polypyrrole and polyaniline. Coating of polypyrrole on textiles hasbeen done by Jolly et al. [34] in a one-step process, in an aqueous solutioncontaining the monomer, FeCl3 oxidant and napthalene sulphonic acid dopant,at a reaction temperature of 5–10◦C. Polymerization occurs at the surface ofthe textile, and each fiber is coated with a homogeneous polypyrrole layer. Thecoated fabric has a surface resistivity of about 10 ohms/sq and has been tried asheating panels for buildings. Effect of aging on the conductivity has also beenstudied at different temperatures. Jin and Gong [35] have deposited polyanilineon polyester fiber and nylon fabric by aniline diffusion and oxidative polymer-ization, using HCl as a dopant. The process consists of immersing the textile inaniline and removing the absorbed aniline from the surface by treatment withhydrochloric acid. The specimens were then treated with aqueous aniline hy-drochloride solution, followed by oxidation with ammonium persulphate, andwashed. Aniline diffusion and use of HCl as dopant enhanced the conductivityand adhesion of the coating. Polyaniline and polypyrrole coatings have alsobeen studied by Trivedi and Dhawan [36] and Gregory and coworkers [37].

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1. P. K. Ramchandran, and N. Raja, Defence Science Journal, 40, 1990, pp. 15–23.2. T. P. Carroll, Journal of Coated Fabrics, vol. 24, 1995, pp. 312–327.3. Janes NBC Protective Equipments, 7th Ed. , T. J. Gander, Ed. , 1994–1995, Jane’s

Informative Group Ltd., U. K., pp. 18–22.4. S. N. Pandey, A. K. Sen, and V. S. Tripathi, Man Made Textiles in India, May, 1994,

pp. 185–188.5. F. A. P. Maggs, P. H. Schwabe, and J. H. William, Nature, 186, 1960, pp. 956–958.6. K. Gurudutt, V. S. Tripathi, and A. K. Sen, Defence Science Journal, 47, 1997, pp.

239–240.7. Textile Horizons, vol. 8, no. 1, 1988, p. 7.8. Textile Horizons, vol. 8; no. 12, 1988, p. 45.9. Kirk Othmer Encyclopedia of Chemical Technology, 3rd Ed., vol. 6, 1979, John

Wiley and Sons, New York, pp. 129–142.10. D. Aitken, S. M. Burkinshaw, J. Griffith, and A. D. Towers, Review of Progress in

Coloration, vol. 26, 1996, pp. 1–8.11. Kirk Othmer Encyclopedia of Chemical Technology, 4th Ed., vol. 16, 1995, John

Wiley and Sons, New York, p. 630.12. Y. Shibahashi, N. Nakasuji, T. Kataoka, H. Inagaki, T. Kito, M. Ozaki, N. Matunami,

N. Ishimura, and K. Fujita, U.S. Patent 4,681,791, 1987.13. T. L. Vigo, and C. M. Frost, Journal of Coated Fabrics, vol. 12, April, 1983,

pp. 243–254.14. T. L. Vigo and C. M. Frost, Textile Research Journal, Dec., 1985, pp. 737–743.15. B. Pause, Journal of Coated Fabrics, vol. 25, July, 1995, pp. 59–67.16. R. Indushekhar, A. Srivastava, and A. K. Sen, Man Made Textiles in India, Dec.,

1996, pp. 449–453.17. S. M. Burkinshaw, G. Hallas, and A. D. Towns, Review of Progress in Coloration,

vol. 26, 1996, pp. 47–53.18. J. Musgrove, Indian Textile Journal, July, 1991, pp. 24–33.19. Technical literature of M/S Barracuda, Sweden.20. J. E. Ford, Textiles, vol. 17, no. 3, 1988, pp. 58–62.21. W. C. Smith, Journal of Coated Fabrics, vol. 17, April, 1988, pp. 242–253.22. Metal coatings, R. D. Athey Jr. , in Coating Technology Handbook, D. Satas, Ed.,

Marcel Dekker, New York, 1991, p. 691.23. Sputtered thin film coatings, B. Aufderheide, in Coating Technology Handbook,

D. Satas, Ed., Marcel Dekker, New York, 1991, p. 217.24. Kirk Othmer Encyclopedia of Chemical Technology, 3rd Ed., vol. 15, 1979, John

Wiley and Sons, New York, p. 264.25. Electroless plating, A. Vaskelis, in Coating Technology Handbook, D. Satas, Ed.,

Marcel Dekker, New York, 1991, p. 187.26. Kirk Othmer Encyclopedia of Chemical Technology, 4th Ed., vol. 9, 1995, John

Wiley and Sons, New York, pp. 198–218.27. R. A. Holmberg, Journal of Coated Fabrics, vol. 18, July, 1988, pp. 64–70.28. Wellington Sears Handbook of Industrial Textiles, S. Adanur, Ed., Technomic Pub-

lishing Co., Inc., Lancaster, PA, 1995.

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29. J. K. Tyagi, Indian Textile Journal, Aug, 1986, pp. 76–80.30. F. Marchini, Journal of Coated Fabrics, vol. 20, Jan., 1991, pp. 153–165.31. L. Temmerman, Journal of Coated Fabrics, vol. 21, Jan., 1992, pp. 191–198.32. K. Bertuleit, Journal of Coated Fabrics, vol. 20, Jan., 1991, pp. 211–215.33. R. Orban, Journal of Coated Fabrics, vol. 18, April, 1989, pp. 246–254.34. R. Jolly, C. Petrescu, J. C. Thiebelmont, J. C. Marechal and F. D. Menneteau, Journal

of Coated Fabrics, vol. 23, Jan., 1994, pp. 228–236.35. X. Jin, and K. Gong, Journal of Coated Fabrics, vol. 26, 1996, pp. 36–43.36. D. C. Trivedi, and S. K. Dhawan, in Proc. of Polymer Symp. 1991, Pune, India,

S. Sivaram, Ed., McGraw Hill, India, 1991.37. R. V. Gregory, W. C. Kimbrell, and H. H. Kuhn, Journal of Coated Fabrics, vol.

20, Jan., 1991, pp. 167–175.

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Test Methods

THERE are a number of tests used to evaluate coated textiles. The basic prin-ciples and relevance of the tests typical to coated fabrics are discussed in

this chapter. Tests that are common to uncoated textiles, such as roll character-istic (length, width, and mass), breaking strength, tear resistance, and burstingstrength, have not been included. ASTM-D 751-79 prescribes that all tests areto be carried out after a lapse of at least 16 h between curing and testing. Spec-imens are cut in such a way that no specimen is nearer than one-tenth of thewidth of the fabrics. The conditioning of the specimens is done at specifiedconditions of temperature and humidity depending on the standard used, i.e.,ASTM, BS, Indian Standard, DIN, etc.


Three test specimens of 2500 mm2 areas in circular, rectangular, or squareshapes, are cut, conditioned, and weighed. The result is expressed in g/m2.The coating of these specimens is then removed by selecting a proper strippingsolution for the particular nature of coating. The bulk of the coating is removedmechanically by wetting the specimens with stripping solvent. The specimensare then refluxed with the solvent, washed with acetone, dried, and weighed. Theprocess is repeated until the difference is <1% between successive refluxingand washing. The stripping solvent for PVC is tetrahydrofuran or methyl ethylketone. For natural rubber on cotton, nitrobenzene/xylene is used. PU-coatedfabrics are stripped by 2 N alcoholic KOH. The weight of the base fabric thusobtained is also expressed in g/m2. The coating mass in g/m2 is obtained bysubtracting the weight of the base fabric from that of the coated fabric.

In a rubberized fabric, the rubber hydrocarbon (polymer) content can be de-termined either by indirect method or by direct method. In the indirect method,the nonrubber ingredients are estimated from the contents of acetone, chloro-form, alcoholic potash extracts, and determination of fillers and sulfur contents.

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The rubber content is obtained by difference. This method is applicable for allolefinic rubbers. Direct method covers the determination of specific rubber poly-mer in the product based on the estimation of an element or a functional group.Thus, natural rubber content can be estimated from the acetic acid generatedon its oxidation by chromic acid. In a similar manner, estimation of nitrogen orchlorine by standard techniques permits calculation of nitrile rubber or neoprenecontent, respectively (IS 5915 and 6110).


PVC is generally coated as a dispersion in a solvent by the spread-coatingprocess. After coating, the layer is fused to form a uniform film. During fusion,phase inversion occurs. Proper fusion determines the durability of the coating.Three test specimens of 40 × 25 mm are cut from the roll of the fabric. Thespecimens are immersed for 15 min in acetone at 20◦C (BS) or 30 min at 23◦C(ASTM), and the coating is examined. If there is no cracking or disintegrationof the coating, disregarding surface effect or removal of lacquer, the sample isconsidered to have passed.

For rubberized fabrics, the specimens are immersed in xylol for 2 h at 27◦C,and the coating is examined. The sample is considered to have cured if there isno tackiness in the coating or no detachment from the base fabric (IS 9491).

9.3 BLOCKING (BS 3424, IS 7016 PART 9)

This test is to check the tackiness of the coating at elevated temperature, sothat the coating is not damaged when stored in rolls. In the BS method, twospecimens of 150 × 75 mm are cut from the roll, placed face to face coveringeach other completely, placed in an oven at 60◦C with a 1.5 kg weight pieceplaced over it, covering half the area of the specimen pair, and kept for 15 min.The specimens are then taken out, and a 100 g weight is hooked on the free endof the lower strip. No blocking is reported if the upper strip can be separatedfrom the bottom strip at 25 mm/s rate of pull without lifting the 100 g weightpiece, and there is no visible damage to the surface of the specimen.

In the IS method (based on ISO/DIS 5978-86), six specimens each of 150 ×150 mm are cut. The specimens are piled in three pairs, back to back, back toface, and face to face. The three pairs are placed in such a way that 100 mm2

pile is formed, leaving the rest of the area free. The pile is placed between twoglass plates in an oven at 70◦C. A 5 kg weight piece is placed over the pileassembly. The specimens are taken out after 3 h, cooled at ambient for 3 h, andexamined. No blocking is reported if the specimens can be separated without

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any sign of adhesion. This method examines blocking between the coated anduncoated surfaces.

9.4 COATING ADHESION (BS 3424, ASTM D-751, IS 7016 PART 5)

This test is of importance because if the adhesion is inadequate, separation ofthe coating from the base fabric may occur. As per BS, test specimens of 75 ×200 mm with length perpendicular to the longitudinal axis are cut. If the coatingis thick, i.e., where the strength of the coating is more than the adhesive bondbetween the coating and the fabric, the coating is manually stripped to about50 mm, and the width of specimen is trimmed to 50 mm. The adhesion strengthcan be determined by either a dynamic method or a dead weight method. Inthe dynamic method, the separated plies of the specimens are clamped to thejaws of an autographic strength testing machine with constant rate of traverse.Coating is separated for about 100 mm by setting the traverse jaw in motion.The adhesion strength is obtained from the load required to separate the coatinglayer.

The dead weight test apparatus consists of two grips, the top fixed to a rigidsupport and the bottom free, capable of accepting dead loads of 200 g units.The separated plies of the specimen are attached to the two grips, and deadweight is gradually placed on the lower jaw until separation occurs within aspecified rate (5 mm in 5 min). This load is recorded. If the thickness of thecoating is thin, two specimens are bonded face to face by an adhesive systemleaving 50 mm free. For vinyl coating, the adhesive may be a solution of PVCresin in tetrahydrofuran, and for PU coating, suitable PU adhesive is taken.At the adhesion line of the two specimens, one layer of fabric and both layersof coating are cut and manually stripped to a specified distance. The adhesionstrength is determined as above by fixing one layer of base cloth in the fixedjaw and the composite layer of two coatings and base cloth in the movable jaw.

ASTM and IS procedures are similar except that they specify only a dynamictensile testing machine for adhesion and that the shape of the peeled layer ofcoating of the specimen is in the form of a tapered tongue (Figure 9.1).

9.5 ACCELERATED AGING (BS 3424, IS 7016 PART 8 BASEDON ISO/R 1419-1970)

Two methods are prescribed for accelerated aging: oven method and oxygenpressure method. In oven aging, specimens are heated in an air oven at 70◦Cor 100◦C, as required, for 168 h. After the exposure, the nature of the coatingis observed for any sign of softening, stiffening, brittleness, or sticking. Deter-mination of a physical property before and after aging permits the calculation

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Figure 9.1 Dimensions and cutting line of test piece (IS 7016 Part V). Cut A extends from oneend of the test piece to within 25 mm of the other end. Cut B extends to within 50 mm of that end.Cut C is a diagonal cut joining the ends of cut A and B (all dimensions in mm). (Adapted withpermission from Bureau of Indian Standards.)

of percentage loss of the physical property on aging. In the oxygen pressureaging test, the specimens are subjected to elevated pressure and temperature byhanging them vertically in a pressure chamber of stainless steel at an oxygenpressure of 2000 kN/m2 and 70◦C for 24 h.

For vinyl-coated fabrics, estimation of loss of plasticizer is of great impor-tance as it gives an idea of the durability of coating on weathering of thesefabrics. ASTM-D1203 describes estimation of loss of volatiles from coatedfabrics under defined conditions of time and temperature using active carbon asthe immersion medium. In the direct contact method (A), three specimens areplaced in a covered container with alternate layers of active carbon of specifiedparticle size. The container is heated in an oven at 70◦C for 24 h. The weightloss of the specimens is measured. Method B, known as the wire cage method,is similar to method A, except that the specimens are placed in the annular spaceof a cylindrical metal cage made of bronze gauze and are not in direct contactwith active carbon. The wire cage method is similar to the BS method. In theIS method (IS 1259), the loss of mass as volatiles is estimated by exposing testspecimens at 100◦C for 24 h in an air oven.

For elastomer-coated fabrics, ASTM D3041 describes an ozone crackingtest. The specimens are under strain by placing them around a mandrel andare exposed in a ozone test chamber containing an atmosphere of ozone andair (50 mPa partial pressure of ozone), and at a temperature of 40◦C. Thespecimens are examined for cracks by magnifying glass after exposure for aspecified duration of time.

A stringent accelerated aging test has been given for inflatable restraint (airbag for automobiles) fabric in ASTM D 5427-95. The fabric specimens areevaluated for selected physical properties after cyclic, heat, humidity, and orozone aging. In cyclic aging, the specimens are aged in a specified cycle oftemperature and humidity conditions. Typical aging cycle conditions are (a)−40◦C, ambient RH for 3 h; (b) 22◦C, 70% RH for 2 h; and (c) 107◦C, ambientRH, for 3 h. The conditions for heat aging, humidity aging, and ozone aging

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Figure 9.2 Apparatus for flexibility determination (IS 7016 Part 11) (all dimensions in mm).(Adapted with permission from Bureau of Indian Standards.)

are 105◦C for 400 h; 80 ◦C, 95% RH for 336 h; and 40 ◦C, 65% RH, 100 pphmozone concentration for 168 h, respectively.


This simple test measures the flexibility of rubber- or plastic-coated fabric.Rectangular strips of 600 mm × 100 mm are cut. A loop is formed from thestrip and placed on a horizontal plane by superposing the two ends that are heldin place under a steel bar (see Figure 9.2). The height of the loop is measured,which gives an idea of the flexibility of the fabric. The lower the loop height,the greater the flexibility, and vice versa.

9.7 DAMAGE DUE TO FLEXING (BS3424, IS 7016 PART 4,ASTM D 2097)

Accelerated flexing of coated fabrics gives useful information of the durabil-ity of the coating in actual use. One of the common methods described in theBS/IS standards for flex testing is the De Mattia method. The apparatus consistsof pairs of flat grips. The grips of each pair are positioned vertically one over theother, and one grip is capable of reciprocating motion in a vertical plane. Thetraverse distance of the grips in open and closed positions is 57 mm. The rate ofreciprocating motion of the grips is specified (300 cycles/min). Test specimensof 45 × 125 mm are cut with length in longitudinal and cross directions. Eachtest specimen is folded with coating outwards along lines 15 mm from each

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of the longer sides and to a width of 15 mm. The specimens are then clampedbetween the grips of the equipment and flexing is carried out for specified cycles(∼100,000 cycles). The specimens after flexing are examined for number ofcracks, their severity, and delamination.

ASTM D 2097 is meant for upholstery leather but has been adopted for vinylupholstery fabric (ASTM D-3690). The testing is carried out in Newark-typeflexing machine containing a pair of pistons. One piston of the pair is stationary,and the other is movable. The piston moves with a reciprocating motion in ahorizontal plane at 500 rpm with a stroke of 32 mm. The closed position ofthe piston is adjusted to 15 times the thickness of the specimens. The sizes oftest specimens are 76 × 114 mm and are clamped on the pair of pistons in acylindrical shape. After a predetermined number of cycles, the fabric is visuallyexamined for cracks.


The abrasion resistance of a coated fabric is determined by abrading thecoated surface of the fabric with an abrader. Measurement of mass loss afterabrasion gives an idea about the abrasion resistance of the coating. In the ASTMmethod, a revolving double-headed platform (RPDH) abrader is used. Circulartest specimens of 110 mm diameter are cut and placed with the coated side upon a specimen holder affixed on a rotating platform. The platform is rotated ina circular motion at 70 rpm. The abrasion is done by two abrasive wheels madeof a specified material that are attached to the free end of a pair of pivoted arms.The abrasive wheels rest on the specimen in a manner that a vertical force isapplied on the specimen by them. The force can be increased by the additionof weights. The abrasion occurs due to friction between the rotating specimenand the abrasive wheels. The loose abraded particles are removed from thespecimen by vacuum cleaner. From the mass loss of the specimen for specifiedcycles of operation, the mass loss per revolution is estimated.

The BS specifies the Martindale abrasion tester for testing expanded PVCcoating. The machine consists of a rectangular base plate on which are mountedfour circular discs covered with specified silicon carbide paper. A top platecontaining a four-specimen holder rests on the center of the abrading discs. Fourcircular test specimens are cut and placed on the specimen holder, with coatedsurface facing the abrading disc. A specified load is applied on the specimensby placing desired weights. The abrasion occurs due to the rotation of the plateholding the specimens, such that the specimen rubs against the abrading disc ina definite pattern. The pattern traced by the plate is similar to Lissajou’s figures,i.e., it changes from a circle to gradually narrowing ellipses to a straight linefollowed by gradually widening ellipses to a circle. After a specified numberof cycles, the exposure of the cellular layer of the coated fabric is noted.

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Figure 9.3 Apparatus for measuring colorfastness to rubbing (IS 1259): (A) abrader member, (B)glass plate, and (C, D) grips. (Adapted with permission from Bureau of Indian Standards.)


The test essentially consists of rubbing the coated fabric specimens withwhite fabric with a specified load and number of cycles and examining the stainimparted, if any, on the white fabric from the test specimen. The test is alsoknown as colorfastness to crocking and is carried out in a crockmeter. Test spec-imens of 230 × 50 mm are cut from the roll and mounted with coated side up ona flat glass surface. A circular piece of bleached white fabric of 25 mm diameteris affixed to a circular brass abrading peg of 16 mm diameter. The abrading pegis fixed to a pivot by an arm. The peg is imparted a reciprocating motion ina straight line parallel to the surface of the test specimen, either manually ormechanically, with a stroke of 100 mm at a rate of 15 cycles/min and is loadedin a manner to exert 0.5 kgf on the test piece. In dry rubbing, the staining of thecotton fabric is examined after ten abrading cycles and compared with the grayscale. In wet rubbing, the cotton fabric is wetted by diluted soap-soda solutionprior to rubbing. Apparatus specified in the IS standard is shown in Figure 9.3.


Coated fabrics are used in many applications requiring low temperature flex-ing. This method is meant for evaluation of the ability of rubber- and plastic-coated fabrics to resist the effect of low temperature, when subjected to bending.

Three test pieces of 25 × 100 mm are cut from the rolls. Each specimen afterconditioning is placed between a pair of glass plates, to prevent curling, and thenplaced into a low temperature cabinet maintained at the specified temperature.A bending jig for bending the sample after exposure is also placed in the cabinet.The bending jig consists of two rectangular aluminium blocks. The blocks areconnected by a hinge and aligned in a straight line. The two blocks are mountedin a frame at an angle of 60 ◦ from the horizontal (Figure 9.4). To the top blockis attached a 250 g weight. A release mechanism folds the hinge at 180◦ sothat the upper plate with weight falls free for bending the specimen. After the

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Figure 9.4 Bending jig for low temperature bend test. (Adapted with permission from Bureau ofIndian Standards.)

end of the exposure period (4 h), sample specimen is placed in the jig and bentwithin the cabinet itself by the release mechanism. The samples are then takenout, folded at 180◦, and examined for cracks and their severity.


This is another test to determine the applicability of rubber- and plastic-coated fabrics at low temperatures. By this method, the lowest temperatureat which the fabrics do not exhibit cracks in the coating when subjected tospecified impact conditions is measured. In the ASTM method, test specimensof 6.4 × 40 mm are die punched, conditioned, and one end is clamped in aspecimen clamp designed to hold the specimen as a cantilever beam, such that

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Figure 9.5 Test piece holder and striker in low temperature impact test. *Refers to Table 1 of IS7016 pt. 14 specifying clearance of striking arm and test piece clamps for test pieces of differentthicknesses (all dimensions in mm). (Adapted with permission from Bureau of Indian Standards.)

the length extending from the clamp is 25 mm. A solenoid-activated strikeris positioned on top of the specimen to impart an impact on it. The radius ofthe striking arm, the position of strike at the specimen from the clamp, thetraverse distance of the striker, and the speed of traverse are specified. The testpiece holder and striker specified in IS are shown in Figure 9.5. The cold cracktemperature varies with the rate of folding, and as such, it is essential to fix theimpact velocity at 2 m/s. The test assembly (specimen, clamp, and striker) isimmersed in a low temperature bath of methanol, silicone, or other suitable heattransfer fluid, or in a refrigerated cold cabinet. After temperature equilibration,impact is applied on the specimen through the striker and cracks in the coatingexamined by taking out the specimen. The temperature of the bath is loweredby intervals at 10◦C until the specimen fails. The temperature of the bath is thenraised by 1◦C intervals until the specimen passes. The temperature 1◦C belowthis point is the cold crack temperature.

In the BS method, a folded test piece in the form of a loop is placed on an anviland immersed in a low temperature bath by a holder. Impact is provided by aspring-actuated hammer of specified weight. The impact velocity is maintainedat 2 m/s. Cold crack temperature is determined in a similar manner.

9.12 CONE TEST (IS 7941)

This is a test for the waterproofness of the fabric that is locally stressed.A circular/rectangular specimen of fabric is folded twice and then opened toobtain a cone with the coated side inward. At the tip of the cone, the material

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is folded so sharp that the coating is heavily stressed. The cone is then put ina wire cone and, in turn, placed into a glass funnel and filled with a specifiedquantity of water. There should not be any penetration of liquid, as well as nowetting of the outer surface of the specimen cone after 18 h.


This test is of great relevance for coated fabrics because it evaluates thecontinuity of coating film and its resistance to water penetration, which is animportant property of certain fabrics, particularly those used as rainwear, covers,and inflatables. ASTM describes two methods. Method A uses a Mullen-typehydrostatic tester, and in method B, pressure is applied by a rising column ofwater.

In a Mullen-type hydrostatic tester, a test specimen is clamped between twocircular clamps having an aperture of 31.2 mm diameter. Hydraulic pressure isapplied to the underside of the specimen by means of a piston forcing water intoa pressure chamber. The pressure is measured by a Bourdon’s gauge. Duringtesting, steadily increasing pressure is applied on the specimen, and the pressureis noted when water first appears through the specimen. Alternately, a specifiedpressure is applied on the specimen for 5 min and appearance of water throughthe fabric is noted. A specimen is considered to have passed if there is no leakageof water at that pressure. Method A is not applicable for fabrics having waterresistance less than 35 kPa pressure.

In method B, the test specimen is mounted on a ring with a conical bottomwith the coated surface in contact with water. The specimen is clamped byplacing a dome-shaped movable water chamber at the top. The water chamberhas a water inlet and a vent. A water leveler consisting of a water inlet, a wateroutlet, and an overflow pipe is attached to the inlet of the chamber and is themeans for setting the head of water. The overflow pipe regulates the level ofwater. During test, the head of water is increased by raising the water levelerby a motorized system, at the rate of 1 cm/s. The pressure at which the firstdrop appears through the underside of the specimen is noted. In an alternativeprocedure, the head of water is kept steady for a specified period, and leakage,if any, is noted. The low pressure method of BS, IS, and the hydrostatic pressuretest of AATCC 127 are similar.


The air permeability of coated fabric is low. This method is used primarily forporous discontinuous coating of breathable fabric. Air permeability is definedas the volume in milliliters of air that passes through the fabric per second, per

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cm2, at a pressure of 1 cm head of water. The apparatus described in the standardconsists of a specimen holder, in which the specimen is clamped between twoflanges having an orifice of 25 mm diameter (a test area of 5.07 cm2). Air issucked through the test specimen by means of a vacuum pump. The rate of flowis adjusted by a series valve and a bypass valve, setting the pressure drop of1 cm water head, across the fabric which is indicated by a dial gauge. The rateof flow of air is measured by an appropriate rotameter when steady pressuredrop of 1 cm head of water is achieved. The apparatus is based on the ShirleyInstitute, U.K., air permeability apparatus.


Water vapor permeability is an important parameter of breathable fabric, asit gives an idea about the comfort property of the fabric. The ASTM methodis, however, a general method for all materials, like paper, plastic, wood, etc.,and not specifically for textiles. Two methods are described, viz., the desiccantmethod and the water method. The details of the two methods are describedbelow.


For the test, a shallow test dish is taken. The weight and size of the dish shouldbe such as that can be weighed in an analytical balance, having a mouth of atleast 3000 mm2 area. The dish should have a ledge around the mouth to fix thetest specimen. A layer (>12 mm) of anhydrous calcium chloride of specifiedmesh size is filled in the dish within 6 mm of the specimen. The mouth of thetest dish is covered with test specimen, and the edges are sealed by molten wax.The whole assembly is weighed and placed in an air circulated, temperatureand humidity controlled test chamber. The temperature of the chamber can bemaintained at a selected value, but a temperature of 32◦C is recommended.The humidity of the chamber is maintained at 50% or 90% RH depending onthe environment desired. The weight gain of the test assembly is measuredperiodically, and at least 10 data points are taken during the duration of the test.A plot of weight against elapsed time is drawn. The slope of the straight lineplot gives G/t (where G is weight change in gms and t is time in h). The watervapor transmission is given by WVT (g/h/m2) = (G/t)A (A is the area of theexposed specimen in m2).


In this procedure, the test dish is filled with distilled water to a depth of3–5 mm, with air gap of about 20 mm from the specimen. The procedure is the

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same as that for the desiccant method. The weight loss is measured periodically.If the barrier material is expected to be in contact with water in service, the dishis inverted during the test.

The BS 7209 method for estimation of water vapor permeability index %(I ), for breathable fabric, is similar to the water method of ASTM. However, inthis specification, the test specimens are tested along with a specified referencefabric, for water vapor permeability, and from the ratio of their water vaporpermeability, I is calculated. The reference fabric is made of monofilamenthigh tenacity woven polyester yarn of 32 µm diameter having an open areaof 12.5%. As per the procedure, test specimens/reference fabric are sealedover the open mouth of a test dish, with cover ring of specified dimensions,containing distilled water. The quantity of water is adjusted to maintain a stillair of 10 ± 1 mm between the underside of the specimen and the surface ofthe water. A sample support placed on the mouth of the dish prevents saggingof the specimen and the resultant change in the depth of the still air layer. Thewhole test assembly is placed on a rotating turntable. The turntable with the testassemblies are, in turn, placed in an environmental chamber, maintained at 65%RH, and 20◦C temperature. The turntable is rotated at a slow specified rate toavoid formation of a still air layer above the dish, care being taken that the depthof still air is not altered inside the dish due to rotation. The test assemblies areweighed after a period of ∼1 h to permit equilibration of water vapor gradientin each assembly. Dishes are then placed back on the turntable in the chamber,and the test is continued for a period of ∼16 h, after which the assemblies arereweighed. From the loss of mass of the assemblies between the two weighings,the index I is calculated.

Water vapor permeability in g/m2/day = 24M/At

where M = loss of mass in g of assembly in time t (h), and A = area of the testfabric in m2 exposed.

I = {(wvp) f /(wvp)r } × 100

where (wvp) f and (wvp)r are the mean permeability of test specimens and thatof reference fabric, respectively.


This test determines the resistance to permeation of a hazardous liquidthrough protective clothing material under continuous contact. The apparatusconsists of a glass cell with two compartments, with the test specimen placed in

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between. One compartment is filled with the hazardous liquid, while the othercontains a collecting fluid to absorb the permeated material. The test specimenacts as a barrier between the challenge chemical and the collecting medium.The collecting fluid may be a liquid or a gas in which the hazardous liquid isfreely soluble. The chemical permeating through the specimen dissolves in thecollecting fluid and is analyzed continuously or discretely by a suitable analyt-ical technique. Some common techniques are UV, IR, spectrophotometry, gasliquid chromatography, and colorimetry. The barrier property of the material ofthe specimen is evaluated by measuring the breakthrough time (BTT) as wellas permeation rate. The BTT is the time elapsed in seconds between the initialcontact of the chemical with the outside surface of the specimen and the time atwhich the chemical can be detected at the inside surface by the analytical tool.


These tests are meant for evaluating protective clothing against chemicalwarfare agents (NBC clothing) that manifest their effect by absorption throughskin (viz., vesicant and nerve gases as discussed in Chapter 8, section 8.1).Among the various chemical agents, sulfur mustard is known to be the mostpenetrating in nature, and any permeable or impermeable fabric found to beeffective against it ought to give a better or the same degree of protectionagainst other members of the category. There is a lot of variation in the testmethods of various countries. Prints Maurits Lab. TNO, The Netherlands, havedeveloped and standardized various test methods, keeping in view all possiblemodes of exposure. These methods find widest acceptance. Two methods mostcommonly used for evaluation are discussed below.


This test is meant for vapor challenge of permeable clothing. One and one-half cm2 of a complete clothing assembly (NBC overgarment, combat cloth,and underwear) positioned in a glass cell is exposed to an airstream of 5 m/sperpendicular to the fabric. The air (5400 L/h) is contaminated with mustard gasvapor in a concentration of 20 mg/m3. The whole system is at room temperature(20–22◦C) and at a RH of 30–80%. Through the underside, air is sucked at aspeed of about 0.3–0.5 cm/s depending on the resistance to air of the completeassembly. The amount of mustard gas penetrated is collected in a bubbler fromthis airstream. The solvent used in the bubbler to trap the mustard gas vaporis either methylisobutylcarbinol (1–2 ml), when the bubbler is exchanged au-tomatically every hour, or diethylsuccinate (1 ml) when only one bubbler is

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used in 6 hrs. The amount of mustard gas collected in the solvent is determinedby gas liquid chromatography with flame photometric detector. The sample isconsidered to have passed the test if the collected amount of mustard in 6 hrsis <500 mg/min/m3.


This test is for testing the effectiveness of both permeable and imperme-able clothing against liquid challenge. A complete assembly combined with apolyethylene film (0. 015 mm) is positioned on a horizontally oriented glass cell.The specimen is fixed with a glass ring of 3–5 mm height and rubber springs.The exposed surface area of the sample is 1.5 cm2. There is a flow of air parallelto the surface of the test specimen of 0. 5 m/s. A droplet of 1 µL mustard gasis placed onto the outer fabric corresponding with a contamination density of8.3 g /m2. A flow of air (L/h) underneath the polyethylene film transports thepenetrated mustard gas vapor to a bubbler. The vapor is trapped in a solvent asin test 1. The sample is considered to have passed if the amount collected in6 hrs is <4 µg/cm2.


Protective clothing is required for workers in the healthcare profession toprotect them from microorganisms in the body fluids of patients. This is par-ticularly necessary for blood-borne viruses that cause hepatitis (hepatitis B andhepatitis C viruses) and acquired immune deficiency syndrome (human immun-odeficiency virus, HIV). This test method assesses the effectiveness of materialsfor protective clothing used for protection of the wearer against contact withblood-borne pathogens using a simulant microbe suspended in a body fluidsimulant. The test equipment essentially consists of a penetration cell (∼60 mLcapacity), which is filled with a bacteriophage (a virus that infects bacteria),challenge suspension, and is then pressurized by air. The test specimen (mate-rial of protective clothing) acts as a barrier restraining the challenge suspension.Any penetration of the challenge suspension through the test specimen is ob-served visually at the other side of the specimen from the viewing side of thecell, and viral penetration is estimated by microbiological assay.

The challenge suspension consists of φX-174 bacteriophage lysate in a nu-trient broth. The φ-X-174 virus is not pathogenic to humans, but due to its size,similarity serves as a simulant of blood-borne pathogens including hepatitis B,hepatitis C, and HIV. The nutrient broth acts as a body fluid simulant havinga surface tension of 0.042 ± 0.002 N/m. The pressure time sequence specified

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for the test is 0 kPa for 5 min, followed by 13.8 kPa for 1 min and 0 kPa for54 min. Any liquid penetration on the other side of the test specimen during thetest indicates failure of the sample. After the test period, the outer side of thetest specimen is rinsed with a sterile nutrient broth and assayed for test virusby standard procedure. The sample is considered to have passed the test if noφ-X-174 virus is detected in the assay [<1 plaque forming unit (PFU)/ml].


This test is important as electrical resistivity influences the accumulation ofelectrostatic charge of the fabric. Besides, this test is also useful for determin-ing the conducting property of metal-coated fabric. The resistance is measuredby a resistance meter. Two rectangular flat metal plates of suitable size serveas electrodes. Alternatively, two concentric ring electrodes of spacing suitableto the material can be used. The size of the test specimen should be such asto accommodate the width/diameter of the electrodes. After proper condition-ing of the test specimen, the electrodes of the resistant meter are placed onit, ensuring firm contact. The resistance is measured in both length and widthdirections after steady state is reached, on passage of current. The lower readingin each direction is recorded. The resistivity R in ohms per square is calculatedas follows. For parallel electrodes, R = O × W/D (O = measured ohms, W =width of specimen, D = distance between the electrodes). For concentric elec-trodes, R = 2.73O/ log r0/r1 (r0 and r1 are outer and inner radius of electrodes,respectively).


1. Laboratory methods for evaluating protective clothing system against chemicalagents, Mary Jo Waters, Report no. CRDC-SP 84010, CRDC, Aberdeen ProvingGround, MD, U.S.A, 1984.

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Properties of Some CommonPolymer Coatings3

Butyl rubber (isobutene-isoprene copolymers) Good resistance to heat aging, ox-idation, UV light, ozone, and general chemical attack. Low permeability to gases.Servicable temperature range −50 to +125◦C. Difficult to seam. Low to moderatecost.

Hypalon (chlorosulfonated polyethylene) Similar to neoprene. Relatively poor lowtemperature resistance. Moderate cost.

Natural rubber (polyisoprene) Good tensile strength and flexibility. Tear strengthand abrasion resistance improved by reinforcing fillers (e.g., carbon black). Insolublein all organic liquids when vulcanized, but highly swollen by hydrocarbons andchlorinated solvents. Unaffected by dilute acids, alkalis, and water. Susceptible tooxidation; less so to ozone. Contains 2–4% of protein, which enhances susceptibilityto biodegradation. Servicable temperature range −55 to +70◦C. Sewn or glued seamsrequired. Moderate cost.

Neoprene (polychloroprene) Good mechanical properties. Resistant to most chem-icals and organic liquids; swollen by chlorinated and aromatic solvents. Excellentweathering properties. Inferior low temperature properties to those of natural rubber.Upper temperature limit about 120◦C. Low to moderate cost.

Nitrile rubber (acrylonitrile-butadiene copolymers) Similar to natural rubber ex-cept for improved resistance to swelling in organic liquids and improved resistanceto heat, light, and oxidative aging. Moderate cost.

PTFE (polytetrafluoroethylene) Exceptional resistance to chemicals, solvents, heat,oxidation, weathering, and microorganisms. Excellent electrical and nonstick prop-erties. Difficult to seam. Serviceable temperature range −70 to +250◦C. Very highcost.

PU (polyurethanes) Very variable compositions; properties range from hard, inflexi-ble plastics to soft, elastic coatings. Plasticizers not required. Some grades have goodresistance to fuels and oils. Excellent strength and resistance to tearing and abrasion.Thermoplastic grades available. Moderate to high cost.

3Adapted with permission from G. R. Lomax Textiles, vol. 14, no. 2. 1985 c© Shirley Institute,U.K.

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PVC (polyvinyl chloride) Naturally rigid material; requires careful formulating toproduce durable, flexible coatings. High plasticizer content (up to 40% by weight).Good chemical properties, although solvents tend to extract plasticizers and stiffenthe polymer. Good weathering properties and flame resistance. Poor low temperatureperformance, unless special plasticizers are used. Thermoplastic and can thereforebe seamed by hot air, radio-frequency, and ultrasonic welding techniques. Low cost.

PVDC (polyvinylidene chloride) Similar to PVC. Better flame resistance. Low per-meability to gases. Low to moderate cost.

SBR (styrene butadiene rubber) Similar to natural rubber except for improved flexand abrasion resistance, particularly under hot, dry conditions. Inferior tear resistanceand serviceable temperature range. Resistant to biodegradation. Moderate cost.

Silicone rubbers (polysiloxanes) Inferior mechanical properties to normal rubbers.Resistant to most chemicals except concentrated acids and alkalis. Resistant to oxida-tion, aging, and microorgansims. Relatively high permeability to gases. Serviceabletemperature range −60 to 200◦C. Tasteless, odorless, and physiologically inert. Dif-ficult to seam. High cost.

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Typical Formulation of CoatingCompounds



PVC polymer: (E, K value 68–70) 100Stabilizer: liquid, Ca/Zn containing (e.g., Irgastab CZ 57) 1.5–3.0 phrCostabilizer: epoxidized soya bean oil 6.0–8.0 phrPlasticizer: DOP 85 phrFiller: whiting 20 phr


PVC polymer (E, K value 68–70) 100Stabilizer/activator: liquid, Zn containing (e.g., Irgastab 1.5–2.5 phr

ABC2)Costabilizer: epoxidized soya bean oil 6.0–8.0 phrPlasticizers: DOP 45 phr

BBP 30 phrBlowing agent: azo dicarbonamide (paste 1:1in DOP) 2.5–4.5 phrFiller: whiting 5 phr


PVC polymer (E, K value 70–72) 100Stabilizer: liquid Ba/Cd/Zn Complex (e.g., Irgastab BC 206) 1.5–2.5 phrCostabilizer: epoxidized soya bean oil 5.0 phrPlasticizer: DOP 52 phr

4Reproduced with permission from PVC Plastics by W. V. Titow. c© Kluwer Academic Publishers,the Netherlands.

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Pigment: TiO2 0.0–3.0 phrFiller: whiting 0.0–10.0 phrColorant: as required

POLYCHLOROPRENE COMPOUND FOR FLOATS, RAFTS,ETC.—FABRIC POLYAMIDE (Courtesy India Waterproofing andDyeing Works, 13 Brabourne Road, Calcutta, India)

(1) Bayprene 110 100.0 (polychloroprene rubber) Bayer(2) Magnesia 4.0(3) Stearic acid 0.5(4) MBTS 1.0 (dibenzthiazyl disulfide)(5) Nonox DN 1.5 (phenyl-β-naphthyl amine)(6) Accinox 4010 NA 0.5 (N-isopropyl N-phenyl-p-phenylene diamine)(7) FEF black 25.0 (fine extrusion furnace black)(8) Silica 20.0(9) DBP 8.0 (dibutyl phthalate)

(10) Zinc oxide 5.0(11) NA 22 0.75 (ethylene thiourea)

RUBBER COMPOUND FOR POTABLE WATER TANKS—FABRICPOLYAMIDE (Courtesy India Waterproofing and Dyeing Works, 13Brabourne Road, Calcutta, India)

(1) Hypalon 45 100.0 (chloro sulfonated polyethylene) Dupont(2) China clay 100.0(3) Magnesia extra light 2.0(4) SRF black 0.5 (semi-reinforcing furnace black)


(1) BIIR 100.0 (bromobutyl rubber 2244—Polysar)(2) CR 20.0 (neoprene WM1—Dupont)(3) Chlorinated paraffin wax 5.0 (58% chlorine content)(4) Stearic acid 1.0(5) PBN 1.0 (phenyl-β-naphthyl amine)(6) Zinc oxide 10.0

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(7) Antimony oxide 10.0(8) Saytax 20.0 (decabromo diphenyl ether)(9) Chlorinated polyethylene 5.0

(10) Magnesium oxide 4.0(11) Sulfur 1.5(12) TMTD 1.5 (tetramethyl thiuram disulfide)(13) MBTS 1.5 (dibenzyl thiazyl disulfide)(14) ZDC 0.5 (zinc diethyl dithiocarbamate)

POLYURETHANE FORMULATION FOR TRANSFER COATING(Courtesy M/S Entremonde Polycoater Ltd., Mumbai, India)

Solution A top coat Impranil C granules 1.0Methyl ethyl ketone 2.5

Solution B tie coat 5% Imprafix TH in solution AImpranil C, an aromatic polyester polyurethane (Bayer)Imprafix TH, a cross-linking agent (Bayer)


SU 5001 1.0Isopropyl alcohol + toluene (1:1) 0.5Pigments if required as desiredSU5001 is an aliphatic polyester polyurethane of Stahl GB U.K


a. BF Goodrich 54630 (polyether)b. BF Goodrich 54620 (polyester)Resins can be pigmented if necessary

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